LeetCode 54 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:

1, The left subtree of a node contains only nodes with keys less than the node's key.

2, The right subtree of a node contains only nodes with keys greater than the node's key.

3, Both the left and right subtrees must also be binary search trees.

分析：

性质已经描述清楚了，换成递归思维就是：

1，左孩子值小于当前值；

2，右孩子值大于当前值；

3，左子树全部值小于当前值；

4，右子树全部值大于当前值。

前两条好满足，后两条怎么办？

可以维持两个边界，某棵子树的所有节点应该在这个范围里，否则，应该返回false.

/**
* 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);

}

public boolean isValidBST(TreeNode node, int min, int max){
if(node == null)
return true;
if(node.val>min && node.val<max && isValidBST(node.left, min, node.val)
&& isValidBST(node.right, node.val, max) )
return true;
else
return false;
}
}