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leetcode-98.+Validate+Binary+Search+Tree

2016-12-26 10:08 本站整理 浏览(5)

leetcode-98. Validate Binary Search Tree

题目: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.

这题本身思路很简单,但是有一个问题我始终解决不了就是当节点是Integer.MIN_VALUE或者Integer.MAX_VALUE的时候。当时做的时候错了很多次,后来看答案才发现原来题目用的是long- -。。确实用long就没有这种问题了。如果在int范围内还是要做很多复杂的判断的。确实是一种典型的解决Integer范围问题的方法,之前也见过很多次,以后需要注意。

[code]/**
 * Definition for a binary tree node.
 * 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, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    public boolean isValidBST(TreeNode root, long minVal, long maxVal) {
        if (root == null) return true;
        if (root.val >= maxVal || root.val <= minVal) return false;
        return isValidBST(root.left, minVal, root.val) && isValidBST(root.right, root.val, maxVal);
    }
}