LeetCode之Validate Binary Search Tree

/*采用中序遍历，判断两个相邻元素是否满足递增规律即可。*/
class Solution {
public:
bool isValidBST(TreeNode* root) {
if(root == nullptr) return true;
TreeNode *pre(nullptr), *cur(root);
stack<TreeNode*> s;
while(!s.empty() || cur != nullptr){
while(cur != nullptr){
s.push(cur);
cur = cur->left;
}
if(!s.empty()){
cur = s.top();
s.pop();
if(pre != nullptr && pre->val >= cur->val) return false;
pre = cur;
cur = cur->right;
}
}
return true;
}
};