Validate Binary Search Tree

2018-01-02

98. Validate Binary Search Tree

判断一个树是不是有效的二叉搜索树

Difficulty: Medium

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:

1
2
3

  2
 / \
1   3

Binary tree [2,1,3], return true.

Example 2:

1
2
3

  1
 / \
2   3

Binary tree [1,2,3], return false.

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public long MAX = 2147483648L;
    public long MIN = -2147483649L;
    public boolean isValidBST(TreeNode root) {
        if(root==null)return true;
      	// 左右子树空间不同
        return dfs(MIN, root.val, root.left) && dfs(root.val, MAX, root.right);
    }
    
    public boolean dfs(long min, long max, TreeNode node){
        //System.out.println(min+":"+max);
        if(null==node)return true;
        if(node.val>=max || node.val<=min)return false;
        
        return dfs(min, node.val, node.left) && dfs(node.val, max, node.right); 
    }
}