Recover Binary Search Tree

Q:

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

A:

中序遍历，比较前后元素大小

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        TreeNode* cur = root;
        stack<TreeNode*> s;
        int pre = -0x7fffffff;
        while (cur || !s.empty()) {
            while (cur) {
                s.push(cur);
                cur = cur->left;
            }
            if (!s.empty()) {
                cur = s.top();
                if (cur->val <= pre) return false;
                pre = cur->val;
                cur = cur->right;
                s.pop();
            }
        }
        return true;
    }
};