leetcode/98.Validate Binary Search Tree/98.cpp at master · arkingc/leetcode · GitHub

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        int *prev = nullptr;
        return isValidBST(root,&prev);
    }

    bool isValidBST(TreeNode* node,int **pprev){
        if(!node)
            return true;

        bool res = isValidBST(node->left,pprev);

        if(*pprev)
            res = res && (node->val > **pprev);
        *pprev = &(node->val);

        res = res && isValidBST(node->right,pprev);

        return res;
    }
};