LeetCode | 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.

这题两种写法：

1. 采用LeetCode | Recover Binary Search Tree中的思路：利用中序遍历结果的单调递增特性进行判断（解法1）；

2. 利用子树上下界的限制进行递归判断（解法2）

解法1

public class ValidateBinarySearchTree {
	private TreeNode pre;

	public boolean isValidBST(TreeNode root) {
		pre = null;
		return inorder(root);
	}

	private boolean inorder(TreeNode root) {
		if (root == null) {
			return true;
		}
		if (!inorder(root.left)) {
			return false;
		}
		if (pre != null && pre.val >= root.val) {
			return false;
		}
		pre = root;
		return inorder(root.right);
	}
}

public class ValidateBinarySearchTree {
	public boolean isValidBST(TreeNode root) {
		return solve(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
	}

	private boolean solve(TreeNode root, int min, int max) {
		if (root == null) {
			return true;
		}
		if (root.val > min && root.val < max) {
			return solve(root.left, min, root.val)
					&& solve(root.right, root.val, max);
		}
		return false;
	}
}

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LeetCode | Validate Binary Search Tree

原文：http://blog.csdn.net/perfect8886/article/details/20284359