LeetCode 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

Binary tree [2,1,3], return true.

Example 2:

    1   / \  2   3

Binary tree [1,2,3], return false.

我们知道中序遍历BST就可以得道一个从小到大排列输出的结果，利用这个性质，每次记录上一个中序遍历的值，和当前输出值比较。不满足上个值小于当前值就是错误的。

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

时间复杂度O(N)，N为节点个数。