leetcode JAVA Validate Binary Search Tree 难度系数3 3.35

原创于 2014-02-05 11:22:51 发布 · 853 阅读

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本文介绍了一种检查给定二叉树是否为有效二叉搜索树的方法。通过递归方式确认每个节点的值是否在其合法范围内，并且左右子树也必须符合二叉搜索树的定义。

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Question:

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.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
     public boolean isValidBST(TreeNode root) {
        return isValidBST(root,Integer.MIN_VALUE,Integer.MAX_VALUE);
    }

	private boolean isValidBST(TreeNode root, int lower, int upper) {
		if(root==null) return true;
		return root.val>lower&&root.val<upper&&isValidBST(root.left, lower, root.val)&&isValidBST(root.right, root.val, upper);
	}
}