98. 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:

• 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。

public class Solution {
    public boolean isValidBST(TreeNode root) {
        return vilidateChild(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    
    public boolean vilidateChild(TreeNode node, long min, long max) {
        if (node == null) return true;
        if (node.val < max && node.val > min) {
            return vilidateChild(node.left, min, node.val) && vilidateChild(node.right, node.val, max);
        } else {
            return false;
        }
    }
}