98. 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

Input: [2,1,3]
Output: true
Example 2:

5

/ \
1 4
/ \
3 6

Input: [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

Solution

非递归中序遍历,所有结点满足pre < curr

Complexity

O(N) O(1)

Code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private TreeNode pre;
    private boolean res = true;
    public boolean isValidBST(TreeNode root) {
        helper(root);
        return res;
    }
    public void helper(TreeNode root){
        if(root == null)
            return;
        helper(root.left);
        
        if(pre != null && pre.val >= root.val)
            res = false;
        pre = root;
        
        helper(root.right);
    }
}