[LeetCode] 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 than the node's key.
• Both the left and right subtrees must also be binary search trees.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

方法一

class Solution {
public:
    TreeNode *left;//记录左边的节点
    TreeNode *right;//记录右边的节点
    bool check(TreeNode *node){
        if(node == NULL) return true;
        bool b1 = check(node -> left);
        left = right;
        right = node;
        if(left != NULL && left -> val >= right -> val) return false;//当左边的比右边的大则返回false
        bool b2 = check(node -> right);
        return b1 && b2;
    }
    bool isValidBST(TreeNode *root) {
        left = NULL;
        right = NULL;
        return check(root);
    }
};

方法二

class Solution {
public:
    bool check(TreeNode *node,long long min,long long max){
        if(node == NULL) return true;
        return (node -> val > min && node -> val < max) && check(node -> left,min,node -> val) 
            && check(node -> right,node -> val,max);
    }
    bool isValidBST(TreeNode *root) {
        return check(root,LONG_LONG_MIN,LONG_LONG_MAX);
    }
};