Update Leetcode 题解 - 分治.md

Added a good example of divide and conquer algorithm.

docs/notes/Leetcode 题解 - 分治.md

@@ -1,5 +1,6 @@
 <!-- GFM-TOC -->
 * [1. 给表达式加括号](#1-给表达式加括号)
+* [2. 不同的二叉树II](#2-不同的二叉树II)
 <!-- GFM-TOC -->
 
 
@@ -48,6 +49,48 @@ public List<Integer> diffWaysToCompute(String input) {
 }
 ```
 
+# 2. 不同的二叉树II
+
+[95. Unique Binary Search Trees II (Medium)](https://leetcode.com/problems/unique-binary-search-trees-ii/description/)
+
+```html
+Input: 3
+Output:
+   1         3     3      2      1
+    \       /     /      / \      \
+     3     2     1      1   3      2
+    /     /       \                 \
+   2     1         2                 3
+```
+
+```java
+public List<TreeNode> generateTrees(int n) {
+	return generateSubtrees(1, n);
+}
+
+private List<TreeNode> generateSubtrees(int s, int e) {
+	List<TreeNode> res = new LinkedList<TreeNode>();
+	if (s > e) {
+		res.add(null);
+		return res;
+	}
+
+	for (int i = s; i <= e; ++i) {
+		List<TreeNode> leftSubtrees = generateSubtrees(s, i - 1);
+		List<TreeNode> rightSubtrees = generateSubtrees(i + 1, e);
+
+		for (TreeNode left : leftSubtrees) {
+			for (TreeNode right : rightSubtrees) {
+				TreeNode root = new TreeNode(i);
+				root.left = left;
+				root.right = right;
+				res.add(root);
+			}
+		}
+	}
+	return res;
+}
+```