Validate Binary Search Tree

Validate Binary Search Tree

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.

Example 1:

    2
   / 
  1   3

Binary tree [2,1,3], return true.

Example 2:

    1
   / 
  2   3

Binary tree [1,2,3], return false.

分析： 判断中序遍历的是不是逐渐增大的，如果不是，return false, 如果是，return true。

/**
 * 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) {
        if(root==NULL)
            return true;
        stack<TreeNode*> s;
        TreeNode* cur=root;
        TreeNode* pre=nullptr;
        while(1){
            
            while(cur){
                s.push(cur);
                cur =cur->left;
            }
            if(s.empty())
                break;
            cur = s.top();
            s.pop();
            if(pre!=nullptr && pre->val >= cur->val)
                return false;
            pre = cur;
            cur = cur->right;
        }
        
        return true;
    }
};