19.3.2 [LeetCode 98] Validate Binary Search Tree

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 thanthe node's key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

Input:
    2
   / 
  1   3
Output: true

Example 2:

    5
   / 
  1   4
     / 
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
             is 5 but its right child's value is 4.

class Solution {
public:
    bool minmax(TreeNode*root,pair<int,int>&ret) {
        if (root->left == NULL && root->right == NULL) {
            ret.first = root->val, ret.second = root->val;
            return true;
        }
        int min=root->val, max=root->val;
        if (root->left) {
            if (minmax(root->left, ret) == false)
                return false;
            if (ret.second >= root->val)
                return false;
            min = ret.first;
        }
        if (root->right) {
            if (minmax(root->right, ret) == false)
                return false;
            if (ret.first <= root->val)
                return false;
            max = ret.second;
        }
        ret.first = min, ret.second = max;
        return true;
    }
    bool isValidBST(TreeNode* root) {
        if (!root)return true;
        pair<int, int>a;
        return minmax(root,a);
    }
};