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LeetCode: 98. Validate Binary Search Tree

Posted on Edited on In , Valine: 
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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) {
        foo(root);
        return ret;
    }
private:
    bool ret = true;
    TreeNode* pre=NULL;
    void foo(TreeNode* root)
    {
        if(root==NULL || !ret)
            return;
        foo(root->left);
        if(pre==NULL)
            pre=root;
        else{
            if(pre->val>=root->val)
            {
                ret=false;
                return;
            }
            pre=root;
        }
        foo(root->right);
    }
};