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LeetCode 98. Validate Binary Search Tree

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Question

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.

Explanation

在递归中，除了和根节点比较大小之外，还要考虑到之前的根节点的大小，所以这里设置一个上下限。

Code

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public class Solution {
    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
            }
            return dfs(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
     public boolean dfs(TreeNode root, long low, long up) {
         if (root == null) {
             return true;
         }
         if (root.val >= up || root.val <= low) {
           return false;
       }
      return dfs(root.left, low, root.val)
            && dfs(root.right, root.val, up);
    }
}