面试题 33. 二叉搜索树的后序遍历序列

     5
    / \
   2   6
  / \
 1   3

输入: [1,6,3,2,5]
输出: false

输入: [1,3,2,6,5]
输出: true

class Solution:
    def verifyPostorder(self, postorder: List[int]) -> bool:
        def verify(p1, p2):
            if p1 > p2:
                return True
            pos = p1
            while pos < p2 and postorder[pos] < postorder[p2]:
                pos += 1
            p = pos
            while pos < p2:
                if postorder[pos] < postorder[p2]:
                    return False
                pos += 1
            return verify(p1, p - 1) and verify(p, p2 - 1)
        if not postorder:
            return True
        return verify(0, len(postorder) - 1)

class Solution {
    public boolean verifyPostorder(int[] postorder) {
        int n;
        if (postorder == null || (n = postorder.length) == 0) return true;
        return verify(postorder, 0, n - 1);
    }

    private boolean  verify(int[] postorder, int p1, int p2) {
        if (p1 >= p2) return true;
        int pos = p1;
        while (pos < p2 && postorder[pos] < postorder[p2]) ++pos;
        int p = pos;
        while (pos < p2) {
            if (postorder[pos] < postorder[p2]) return false;
            ++pos;
        }
        return verify(postorder, p1, p - 1) && verify(postorder, p, p2 - 1);
    }
}

/**
 * @param {number[]} postorder
 * @return {boolean}
 */
var verifyPostorder = function (postorder) {
  if (!postorder || postorder.length < 2) return true;
  let mid = 0;
  let root = postorder[postorder.length - 1];
  for (let i = 0; i < postorder.length - 1 && postorder[i] < root; i++) {
    mid++;
  }
  for (let i = mid + 1; i < postorder.length - 1; i++) {
    if (postorder[i] < root) return false;
  }
  return (
    verifyPostorder(postorder.slice(0, mid)) &&
    verifyPostorder(postorder.slice(mid + 1, postorder.length - 1))
  );
};

func verifyPostorder(postorder []int) bool {
    if len(postorder) < 2 {
        return true
    }
    return helper(postorder, 0, len(postorder)-1)
}
// 递归
func helper(postorder []int , left,right int) bool {
    if left >= right {
        return true
    }
    // 最后一位即根
    rootValue := postorder[right]
    // 从左开始往右遍历，直到大于根停止,小于部分是左子树
    i := left
    for i < right && postorder[i] < rootValue {
        i++
    }
    // 剩下部分是右子树，检查是否都大于根值
    for j := i; j < right; j++ {
        if postorder[j] < rootValue {
            return false
        }
    }
    l := helper(postorder,left,i-1) // 检查左子树，左子树i要减一
    r := helper(postorder,i,right-1)// 检查右子树，剔除最后一位是根
    return l && r
}

class Solution {
public:
    bool verifyPostorder(vector<int>& postorder) {
        return verify(postorder, 0, postorder.size() - 1);
    }

    bool verify(vector<int>& postorder, int left, int right) {
        if (left >= right) {
            return true;
        }
        int root = postorder[right], i = left;
        while (postorder[i] < root) {
            ++i;
        }
        int mid = i;
        while (i < right) {
            if (postorder[i] < root) {
                return false;
            }
            ++i;
        }
        return verify(postorder, left, mid - 1) && verify(postorder, mid, right - 1);
    }
};