剑指 Offer 33. 二叉搜索树的后序遍历序列

输入: [1,6,3,2,5]
输出: false

输入: [1,3,2,6,5]
输出: true

class Solution {
    public boolean verifyPostorder(int[] postorder) {
        if(postorder == null){
            return true;
        }

        return f(postorder, 0, postorder.length - 1);
    }

    boolean f(int[] postorder, int i, int j){
        if(i >= j){
            return true;
        }

        int root = postorder[j];
        int p = i;
        // 获取第一个大于或者等于 root 等元素的位置
        while(postorder[p] < root) p++;
        // 判断 p ~ j -1 这个范围是否存在小于root的元素
        for(int k = p; k < j; k++){
            if(postorder[k] < root){
                return false;
            }
        }

        return f(postorder, i, p - 1) && f(postorder, p, j - 1);
    }
}

class Solution(object):
    def verifyPostorder(self, postorder):
        """
        :type postorder: List[int]
        :rtype: bool
        """
        if not postorder:
            return True

        def f(postorder, i, j):
            if i >= j:
                return True
            root = postorder[j]
            p = i
            while postorder[p] < root:
                p += 1
            for k in range(p, j):
                if postorder[k] < root:
                    return False
            return f(postorder, i, p - 1) and f(postorder, p, j - 1)

        return f(postorder, 0, len(postorder) - 1)

class Solution {
public:
    bool verifyPostorder(vector<int>& postorder) {
        if (postorder.empty()) {
            return true;
        }

        function<bool(int, int)> f = [&](int i, int j) {
            if (i >= j) {
                return true;
            }
            int root = postorder[j];
            int p = i;
            while (postorder[p] < root) {
                ++p;
            }
            for (int k = p; k < j; ++k) {
                if (postorder[k] < root) {
                    return false;
                }
            }
            return f(i, p - 1) && f(p, j - 1);
        };

        return f(0, postorder.size() - 1);
    }
};

func verifyPostorder(postorder []int) bool {
    if len(postorder) == 0 {
        return true
    }

    var f func(int, int) bool
    f = func(i, j int) bool {
        if i >= j {
            return true
        }
        root := postorder[j]
        p := i
        for postorder[p] < root {
            p++
        }
        for k := p; k < j; k++ {
            if postorder[k] < root {
                return false
            }
        }
        return f(i, p-1) && f(p, j-1)
    }

    return f(0, len(postorder)-1)
}

/**
 * @param {number[]} postorder
 * @return {boolean}
 */
var verifyPostorder = function (postorder) {
  if (postorder == null) {
    return true;
  }

  return f(postorder, 0, postorder.length - 1);
};

/**
 * @param {number[]} postorder
 * @param {number} i
 * @param {number} j
 * @return {boolean}
 */
var f = function (postorder, i, j) {
  if (i >= j) {
    return true;
  }

  var root = postorder[j];
  var p = i;
  // 获取第一个大于或者等于 root 等元素的位置
  while (postorder[p] < root) p++;
  // 判断 p ~ j -1 这个范围是否存在小于root的元素
  for (var k = p; k < j; k++) {
    if (postorder[k] < root) {
      return false;
    }
  }

  return f(postorder, i, p - 1) && f(postorder, p, j - 1);
};