98[Medimu]: Validate Binary Search Tree

Part1:问题描述

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

    2
   / \
  1   3

Binary tree  [2,1,3] , return true.

Example 2:

    1
   / \
  2   3

Binary tree  [1,2,3] , return false.

Part2:解题思路

  树的题一般都可以用递归和非递归2种方式来做，如果不用递归可以根据特点——中序遍历树的时候，得到的节点的值一定是递增的来写。下面的代码是参考网上递归的方式写的比较简洁。

Part3:代码

class Solution {
public:
    bool isValidBST(TreeNode* root) {
        int min, max;
        return isValidBST(root, &min, &max);
    }
    bool isValidBST(TreeNode* root, int* min, int* max) {
        if (root == NULL) { return true; }
        *min = *max = root->val;
        int tmp;
        if (root->left != NULL) {
            if (!isValidBST(root->left, min, &tmp) || tmp >= root->val) { return false; }
        }
        if (root->right != NULL) {
            if (!isValidBST(root->right, &tmp, max) || tmp <= root->val) { return false;}
        }
        return true;
    }
};