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Solution.java
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Solution.java
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package validatebst;
import datastructures.TreeNode;
/**
* 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
*
* Input: [2,1,3]
* Output: true
* Example 2:
*
* 5
* / \
* 1 4
* / \
* 3 6
*
* Input: [5,1,4,null,null,3,6]
* Output: false
* Explanation: The root node's value is 5 but its right child's value is 4.
*/
class Solution {
public boolean isValidBST(TreeNode root) {
return validator(root, null, null);
}
private boolean validator(TreeNode node, Integer lower, Integer upper) {
if(node == null)
return true;
int val = node.val;
// check upper & lower limits
if(lower != null && val <= lower)
return false;
if(upper != null && val >= upper)
return false;
return validator(node.left, lower, val) && validator(node.right, val, upper);
}
}
/*
Details
Runtime: 0 ms, faster than 100.00% of Java online submissions for Validate Binary Search Tree.
Memory Usage: 40.7 MB, less than 5.58% of Java online submissions for Validate Binary Search Tree.
*/