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2018-04-23 12:11:28 +08:00
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notes/Leetcode 题解.md
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@ -4293,6 +4293,8 @@ class Tuple implements Comparable<Tuple> {
## 链表 ## 链表
链表是空节点，或者有一个值和一个指向下一个链表的指针，因此很多链表问题可以用递归来处理。
**找出两个链表的交点** **找出两个链表的交点**
[Leetcode : 160. Intersection of Two Linked Lists (Easy)](https://leetcode.com/problems/intersection-of-two-linked-lists/description/) [Leetcode : 160. Intersection of Two Linked Lists (Easy)](https://leetcode.com/problems/intersection-of-two-linked-lists/description/)
@ -4305,7 +4307,7 @@ A: a1 → a2
B: b1 → b2 → b3 B: b1 → b2 → b3
``` ```
要求：时间复杂度为 O(n) 空间复杂度为 O(1) 要求：时间复杂度为 O(N) 空间复杂度为 O(1)
设 A 的长度为 a + c，B 的长度为 b + c，其中 c 为尾部公共部分长度，可知 a + c + b = b + c + a。 设 A 的长度为 a + c，B 的长度为 b + c，其中 c 为尾部公共部分长度，可知 a + c + b = b + c + a。
@ -4313,7 +4315,6 @@ B: b1 → b2 → b3
```java ```java
public ListNode getIntersectionNode(ListNode headA, ListNode headB) { public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
if(headA == null || headB == null) return null;
ListNode l1 = headA, l2 = headB; ListNode l1 = headA, l2 = headB;
while(l1 != l2){ while(l1 != l2){
l1 = (l1 == null) ? headB : l1.next; l1 = (l1 == null) ? headB : l1.next;
@ -4329,40 +4330,49 @@ public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
[Leetcode : 206. Reverse Linked List (Easy)](https://leetcode.com/problems/reverse-linked-list/description/) [Leetcode : 206. Reverse Linked List (Easy)](https://leetcode.com/problems/reverse-linked-list/description/)
头插法能够按逆序构建链表。 递归
```java ```java
public ListNode reverseList(ListNode head) { public ListNode reverseList(ListNode head) {
ListNode newHead = null; // 设为 null，作为新链表的结尾 if (head == null || head.next == null) return head;
while(head != null){ ListNode next = head.next;
ListNode nextNode = head.next; ListNode newHead = reverseList(next);
head.next = newHead; next.next = head;
newHead = head; head.next = null;
head = nextNode;
}
return newHead; return newHead;
} }
``` ```
头插法
```java
public ListNode reverseList(ListNode head) {
ListNode newHead = new ListNode(-1);
while (head != null) {
ListNode next = head.next;
head.next = newHead.next;
newHead.next = head;
head = next;
}
return newHead.next;
}
```
**归并两个有序的链表** **归并两个有序的链表**
[Leetcode : 21. Merge Two Sorted Lists (Easy)](https://leetcode.com/problems/merge-two-sorted-lists/description/) [Leetcode : 21. Merge Two Sorted Lists (Easy)](https://leetcode.com/problems/merge-two-sorted-lists/description/)
链表和树一样，可以用递归方式来定义：链表是空节点，或者有一个值和一个指向下一个链表的指针。因此很多链表问题可以用递归来处理。
```java ```java
public ListNode mergeTwoLists(ListNode l1, ListNode l2) { public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if(l1 == null) return l2; if (l1 == null) return l2;
if(l2 == null) return l1; if (l2 == null) return l1;
ListNode newHead = null; if (l1.val < l2.val) {
if(l1.val < l2.val){ l1.next = mergeTwoLists(l1.next, l2);
newHead = l1; return l1;
newHead.next = mergeTwoLists(l1.next, l2); } else {
} else{ l2.next = mergeTwoLists(l1, l2.next);
newHead = l2; return l2;
newHead.next = mergeTwoLists(l1, l2.next);
} }
return newHead;
} }
``` ```
@ -4377,7 +4387,7 @@ Given 1->1->2->3->3, return 1->2->3.
```java ```java
public ListNode deleteDuplicates(ListNode head) { public ListNode deleteDuplicates(ListNode head) {
if(head == null || head.next == null) return head; if (head == null || head.next == null) return head;
head.next = deleteDuplicates(head.next); head.next = deleteDuplicates(head.next);
return head.next != null && head.val == head.next.val ? head.next : head; return head.next != null && head.val == head.next.val ? head.next : head;
} }
@ -4654,7 +4664,7 @@ public ListNode oddEvenList(ListNode head) {
```java ```java
public int maxDepth(TreeNode root) { public int maxDepth(TreeNode root) {
if(root == null) return 0; if (root == null) return 0;
return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1; return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
} }
``` ```
@ -4665,7 +4675,7 @@ public int maxDepth(TreeNode root) {
```java ```java
public TreeNode invertTree(TreeNode root) { public TreeNode invertTree(TreeNode root) {
if(root == null) return null; if (root == null) return null;
TreeNode left = root.left; // 后面的操作会改变 left 指针，因此先保存下来 TreeNode left = root.left; // 后面的操作会改变 left 指针，因此先保存下来
root.left = invertTree(root.right); root.left = invertTree(root.right);
root.right = invertTree(left); root.right = invertTree(left);
@ -4696,9 +4706,9 @@ Merged tree:
```java ```java
public TreeNode mergeTrees(TreeNode t1, TreeNode t2) { public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
if(t1 == null && t2 == null) return null; if (t1 == null && t2 == null) return null;
if(t1 == null) return t2; if (t1 == null) return t2;
if(t2 == null) return t1; if (t2 == null) return t1;
TreeNode root = new TreeNode(t1.val + t2.val); TreeNode root = new TreeNode(t1.val + t2.val);
root.left = mergeTrees(t1.left, t2.left); root.left = mergeTrees(t1.left, t2.left);
root.right = mergeTrees(t1.right, t2.right); root.right = mergeTrees(t1.right, t2.right);
@ -4726,8 +4736,8 @@ return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.
```java ```java
public boolean hasPathSum(TreeNode root, int sum) { public boolean hasPathSum(TreeNode root, int sum) {
if(root == null) return false; if (root == null) return false;
if(root.left == null && root.right == null && root.val == sum) return true; if (root.left == null && root.right == null && root.val == sum) return true;
return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val); return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);
} }
``` ```
@ -4764,14 +4774,61 @@ public int pathSum(TreeNode root, int sum) {
} }
private int pathSumStartWithRoot(TreeNode root, int sum){ private int pathSumStartWithRoot(TreeNode root, int sum){
if(root == null) return 0; if (root == null) return 0;
int ret = 0; int ret = 0;
if(root.val == sum) ret++; if (root.val == sum) ret++;
ret += pathSumStartWithRoot(root.left, sum - root.val) + pathSumStartWithRoot(root.right, sum - root.val); ret += pathSumStartWithRoot(root.left, sum - root.val) + pathSumStartWithRoot(root.right, sum - root.val);
return ret; return ret;
} }
``` ```
**子树**
[Leetcode : 572. Subtree of Another Tree (Easy)](https://leetcode.com/problems/subtree-of-another-tree/description/)
```html
Given tree s:
3
/ \
4 5
/ \
1 2
Given tree t:
4
/ \
1 2
Return true, because t has the same structure and node values with a subtree of s.
Given tree s:
3
/ \
4 5
/ \
1 2
/
0
Given tree t:
4
/ \
1 2
Return false.
```
```java
public boolean isSubtree(TreeNode s, TreeNode t) {
if (s == null) return false;
return isSubtreeWithRoot(s, t) || isSubtree(s.left, t) || isSubtree(s.right, t);
}
private boolean isSubtreeWithRoot(TreeNode s, TreeNode t) {
if (t == null && s == null) return true;
if (t == null || s == null) return false;
if (t.val != s.val) return false;
return isSubtreeWithRoot(s.left, t.left) && isSubtreeWithRoot(s.right, t.right);
}
```
**树的对称** **树的对称**
[Leetcode : 101. Symmetric Tree (Easy)](https://leetcode.com/problems/symmetric-tree/description/) [Leetcode : 101. Symmetric Tree (Easy)](https://leetcode.com/problems/symmetric-tree/description/)
@ -4786,14 +4843,14 @@ private int pathSumStartWithRoot(TreeNode root, int sum){
```java ```java
public boolean isSymmetric(TreeNode root) { public boolean isSymmetric(TreeNode root) {
if(root == null) return true; if (root == null) return true;
return isSymmetric(root.left, root.right); return isSymmetric(root.left, root.right);
} }
private boolean isSymmetric(TreeNode t1, TreeNode t2){ private boolean isSymmetric(TreeNode t1, TreeNode t2){
if(t1 == null && t2 == null) return true; if (t1 == null && t2 == null) return true;
if(t1 == null || t2 == null) return false; if (t1 == null || t2 == null) return false;
if(t1.val != t2.val) return false; if (t1.val != t2.val) return false;
return isSymmetric(t1.left, t2.right) && isSymmetric(t1.right, t2.left); return isSymmetric(t1.left, t2.right) && isSymmetric(t1.right, t2.left);
} }
``` ```
@ -4837,10 +4894,10 @@ public int maxDepth(TreeNode root) {
```java ```java
public int minDepth(TreeNode root) { public int minDepth(TreeNode root) {
if(root == null) return 0; if (root == null) return 0;
int left = minDepth(root.left); int left = minDepth(root.left);
int right = minDepth(root.right); int right = minDepth(root.right);
if(left == 0 || right == 0) return left + right + 1; if (left == 0 || right == 0) return left + right + 1;
return Math.min(left, right) + 1; return Math.min(left, right) + 1;
} }
``` ```
@ -4861,13 +4918,13 @@ There are two left leaves in the binary tree, with values 9 and 15 respectively.
```java ```java
public int sumOfLeftLeaves(TreeNode root) { public int sumOfLeftLeaves(TreeNode root) {
if(root == null) return 0; if (root == null) return 0;
if(isLeaf(root.left)) return root.left.val + sumOfLeftLeaves(root.right); if (isLeaf(root.left)) return root.left.val + sumOfLeftLeaves(root.right);
return sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right); return sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right);
} }
private boolean isLeaf(TreeNode node){ private boolean isLeaf(TreeNode node){
if(node == null) return false; if (node == null) return false;
return node.left == null && node.right == null; return node.left == null && node.right == null;
} }
``` ```
@ -4903,49 +4960,15 @@ Output:
```java ```java
public TreeNode trimBST(TreeNode root, int L, int R) { public TreeNode trimBST(TreeNode root, int L, int R) {
if(root == null) return null; if (root == null) return null;
if(root.val > R) return trimBST(root.left, L, R); if (root.val > R) return trimBST(root.left, L, R);
if(root.val < L) return trimBST(root.right, L, R); if (root.val < L) return trimBST(root.right, L, R);
root.left = trimBST(root.left, L, R); root.left = trimBST(root.left, L, R);
root.right = trimBST(root.right, L, R); root.right = trimBST(root.right, L, R);
return root; return root;
} }
``` ```
**子树**
[Leetcode : 572. Subtree of Another Tree (Easy)](https://leetcode.com/problems/subtree-of-another-tree/description/)
```html
Given tree s:
3
/ \
4 5
/ \
1 2
Given tree t:
4
/ \
1 2
Return true, because t has the same structure and node values with a subtree of s.
```
```java
public boolean isSubtree(TreeNode s, TreeNode t) {
if(s == null && t == null) return true;
if(s == null || t == null) return false;
if(s.val == t.val && isSame(s, t)) return true;
return isSubtree(s.left, t) || isSubtree(s.right, t);
}
private boolean isSame(TreeNode s, TreeNode t){
if(s == null && t == null) return true;
if(s == null || t == null) return false;
if(s.val != t.val) return false;
return isSame(s.left, t.left) && isSame(s.right, t.right);
}
```
**从有序数组中构造二叉查找树** **从有序数组中构造二叉查找树**
[Leetcode : 108. Convert Sorted Array to Binary Search Tree (Easy)](https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/description/) [Leetcode : 108. Convert Sorted Array to Binary Search Tree (Easy)](https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/description/)
@ -4956,7 +4979,7 @@ public TreeNode sortedArrayToBST(int[] nums) {
} }
private TreeNode toBST(int[] nums, int sIdx, int eIdx){ private TreeNode toBST(int[] nums, int sIdx, int eIdx){
if(sIdx > eIdx) return null; if (sIdx > eIdx) return null;
int mIdx = (sIdx + eIdx) / 2; int mIdx = (sIdx + eIdx) / 2;
TreeNode root = new TreeNode(nums[mIdx]); TreeNode root = new TreeNode(nums[mIdx]);
root.left = toBST(nums, sIdx, mIdx - 1); root.left = toBST(nums, sIdx, mIdx - 1);
@ -5016,14 +5039,14 @@ Output: 5
```java ```java
public int findSecondMinimumValue(TreeNode root) { public int findSecondMinimumValue(TreeNode root) {
if(root == null) return -1; if (root == null) return -1;
if(root.left == null && root.right == null) return -1; if (root.left == null && root.right == null) return -1;
int leftVal = root.left.val; int leftVal = root.left.val;
int rightVal = root.right.val; int rightVal = root.right.val;
if(leftVal == root.val) leftVal = findSecondMinimumValue(root.left); if (leftVal == root.val) leftVal = findSecondMinimumValue(root.left);
if(rightVal == root.val) rightVal = findSecondMinimumValue(root.right); if (rightVal == root.val) rightVal = findSecondMinimumValue(root.right);
if(leftVal != -1 && rightVal != -1) return Math.min(leftVal, rightVal); if (leftVal != -1 && rightVal != -1) return Math.min(leftVal, rightVal);
if(leftVal != -1) return leftVal; if (leftVal != -1) return leftVal;
return rightVal; return rightVal;
} }
``` ```
@ -5045,8 +5068,8 @@ For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another exa
```java ```java
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) { public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if(root.val > p.val && root.val > q.val) return lowestCommonAncestor(root.left, p, q); if (root.val > p.val && root.val > q.val) return lowestCommonAncestor(root.left, p, q);
if(root.val < p.val && root.val < q.val) return lowestCommonAncestor(root.right, p, q); if (root.val < p.val && root.val < q.val) return lowestCommonAncestor(root.right, p, q);
return root; return root;
} }
``` ```
@ -5091,13 +5114,14 @@ Output : 2
```java ```java
private int path = 0; private int path = 0;
public int longestUnivaluePath(TreeNode root) { public int longestUnivaluePath(TreeNode root) {
dfs(root); dfs(root);
return path; return path;
} }
private int dfs(TreeNode root){ private int dfs(TreeNode root){
if(root == null) return 0; if (root == null) return 0;
int left = dfs(root.left); int left = dfs(root.left);
int right = dfs(root.right); int right = dfs(root.right);
int leftPath = root.left != null && root.left.val == root.val ? left + 1 : 0; int leftPath = root.left != null && root.left.val == root.val ? left + 1 : 0;
@ -5146,17 +5170,17 @@ public int rob(TreeNode root) {
```java ```java
public List<Double> averageOfLevels(TreeNode root) { public List<Double> averageOfLevels(TreeNode root) {
List<Double> ret = new ArrayList<>(); List<Double> ret = new ArrayList<>();
if(root == null) return ret; if (root == null) return ret;
Queue<TreeNode> queue = new LinkedList<>(); Queue<TreeNode> queue = new LinkedList<>();
queue.add(root); queue.add(root);
while(!queue.isEmpty()){ while (!queue.isEmpty()){
int cnt = queue.size(); int cnt = queue.size();
double sum = 0; double sum = 0;
for(int i = 0; i < cnt; i++){ for (int i = 0; i < cnt; i++){
TreeNode node = queue.poll(); TreeNode node = queue.poll();
sum += node.val; sum += node.val;
if(node.left != null) queue.add(node.left); if (node.left != null) queue.add(node.left);
if(node.right != null) queue.add(node.right); if (node.right != null) queue.add(node.right);
} }
ret.add(sum / cnt); ret.add(sum / cnt);
} }
@ -5187,10 +5211,10 @@ Output:
public int findBottomLeftValue(TreeNode root) { public int findBottomLeftValue(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<>(); Queue<TreeNode> queue = new LinkedList<>();
queue.add(root); queue.add(root);
while(!queue.isEmpty()){ while (!queue.isEmpty()){
root = queue.poll(); root = queue.poll();
if(root.right != null) queue.add(root.right); if (root.right != null) queue.add(root.right);
if(root.left != null) queue.add(root.left); if (root.left != null) queue.add(root.left);
} }
return root.val; return root.val;
} }
@ -5341,17 +5365,17 @@ public boolean findTarget(TreeNode root, int k) {
List<Integer> nums = new ArrayList<>(); List<Integer> nums = new ArrayList<>();
inOrder(root, nums); inOrder(root, nums);
int i = 0, j = nums.size() - 1; int i = 0, j = nums.size() - 1;
while(i < j){ while (i < j){
int sum = nums.get(i) + nums.get(j); int sum = nums.get(i) + nums.get(j);
if(sum == k) return true; if (sum == k) return true;
if(sum < k) i++; if (sum < k) i++;
else j--; else j--;
} }
return false; return false;
} }
private void inOrder(TreeNode root, List<Integer> nums){ private void inOrder(TreeNode root, List<Integer> nums){
if(root == null) return; if (root == null) return;
inOrder(root.left, nums); inOrder(root.left, nums);
nums.add(root.val); nums.add(root.val);
inOrder(root.right, nums); inOrder(root.right, nums);
@ -5386,9 +5410,9 @@ public int getMinimumDifference(TreeNode root) {
} }
private void inorder(TreeNode node){ private void inorder(TreeNode node){
if(node == null) return; if (node == null) return;
inorder(node.left); inorder(node.left);
if(preVal != -1) minDiff = Math.min(minDiff, Math.abs(node.val - preVal)); if (preVal != -1) minDiff = Math.min(minDiff, Math.abs(node.val - preVal));
preVal = node.val; preVal = node.val;
inorder(node.right); inorder(node.right);
} }
@ -5487,13 +5511,13 @@ private void inOrder(TreeNode node) {
```java ```java
public int kthSmallest(TreeNode root, int k) { public int kthSmallest(TreeNode root, int k) {
int leftCnt = count(root.left); int leftCnt = count(root.left);
if(leftCnt == k - 1) return root.val; if (leftCnt == k - 1) return root.val;
if(leftCnt > k - 1) return kthSmallest(root.left, k); if (leftCnt > k - 1) return kthSmallest(root.left, k);
return kthSmallest(root.right, k - leftCnt - 1); return kthSmallest(root.right, k - leftCnt - 1);
} }
private int count(TreeNode node) { private int count(TreeNode node) {
if(node == null) return 0; if (node == null) return 0;
return 1 + count(node.left) + count(node.right); return 1 + count(node.left) + count(node.right);
} }
``` ```