33. 二叉搜索树的后序遍历序列.md

33. 二叉搜索树的后序遍历序列

NowCoder

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。

解题思路

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);
}