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docs/notes/33. 二叉搜索树的后序遍历序列.md

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+# 33. 二叉搜索树的后序遍历序列
+
+[NowCoder](https://www.nowcoder.com/practice/a861533d45854474ac791d90e447bafd?tpId=13&tqId=11176&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking&from=cyc_github)
+
+## 题目描述
+
+输入一个整数数组，判断该数组是不是某二叉搜索树的后序遍历的结果。假设输入的数组的任意两个数字都互不相同。
+
+例如，下图是后序遍历序列 1,3,2 所对应的二叉搜索树。
+
+<img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/13454fa1-23a8-4578-9663-2b13a6af564a.jpg" width="150"/>
+
+## 解题思路
+
+```java
+public boolean VerifySquenceOfBST(int[] sequence) {
+    if (sequence == null || sequence.length == 0)
+        return false;
+    return verify(sequence, 0, sequence.length - 1);
+}
+
+private boolean verify(int[] sequence, int first, int last) {
+    if (last - first <= 1)
+        return true;
+    int rootVal = sequence[last];
+    int cutIndex = first;
+    while (cutIndex < last && sequence[cutIndex] <= rootVal)
+        cutIndex++;
+    for (int i = cutIndex; i < last; i++)
+        if (sequence[i] < rootVal)
+            return false;
+    return verify(sequence, first, cutIndex - 1) && verify(sequence, cutIndex, last - 1);
+}
+```