Algorithm

LeetCode/Validate Binary Search Tree

Problem Summary

Given a binary tree, determine if it is a binary search tree.

Solution

I have wrote about this problem before: https://suzyz.github.io/2017/10/14/validate-binary-search-tree/

Now I would like to implement Solution 2 in that post. That is, carry out an inorder traversal with the help of a stack.

It is relatively harder to code, but not much work actually.

Code

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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) {
        stack<TreeNode *> st;
        TreeNode *pre = NULL;
        while (root != NULL || !st.empty())
        {
            while (root != NULL)
            {
                st.push(root);
                root = root->left;
            }
            TreeNode *cur = st.top();
            st.pop();
            if (pre != NULL && cur->val <= pre->val)
                return false;
            pre = cur;
            root = cur->right;
        }
        return true;
    }
};