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.
• A single node tree is a BST

Example

An example:

2 / \1   4   / \  3   5

The above binary tree is serialized as {2,1,4,#,#,3,5} (in level order).

分析：

解题思路可以根据bst的特性来，对于每个子树，我们给他们一个最小和最大的范围。

1 public class Solution { 2     public boolean isValidBST(TreeNode root) { 3         return vilidateChild(root, Long.MIN_VALUE, Long.MAX_VALUE); 4     } 5      6     public boolean vilidateChild(TreeNode node, long min, long max) { 7         if (node == null) return true; 8         if (node.val < max && node.val > min) { 9             return vilidateChild(node.left, min, node.val) && vilidateChild(node.right, node.val, max);10         } else {11             return false;12         }13     }14 }