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LeetCode: 98. Validate Binary Search Tree

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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);
}
};