【LeetCode】Validate Binary Search Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode *root) {
    stack<TreeNode*> s;
   
    TreeNode* p = root;
    TreeNode* pre = NULL;   // pre保持为p的中序前驱
    while(p || !s.empty())
    {
        if(p)
        {
            s.push(p);
            p = p->left;
        }
        else
        {
            p = s.top(); s.pop();
            if( pre && (p->val <= pre->val) )  return false;  // 不是二叉排序树
            pre = p;   // 记下前驱结点
            p = p->right;
        }
    }
    return true;  // 二叉排序树
    }
};