p98

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LeetCode p98 Validate Binary Search Tree 题解

1.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.

题意：

输入一二叉树，判断它是否为二叉搜索树。

2.解题思路:

见代码。

3.代码

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/**
 * 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) {
        if (root == null)
			return true;

		return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE); //Int会被卡数
	}

	public boolean isValidBST(TreeNode root, Long min, Long max) {
		if (root == null)
			return true;
		if (root.val <= min || root.val >= max)
			return false;
		boolean b1 = root.left == null
				|| (root.left != null && root.left.val < root.val);
		boolean b2 = root.right == null
				|| (root.right != null && root.right.val > root.val);
		if (b1 && b2) {
			return isValidBST(root.left, min, (long) root.val)
					&& isValidBST(root.right, (long) root.val, max);
		}
		return false;
	}
}

4.一些总结：