LeetCode - 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.

Solution 1:

 1 /**

 2  * Definition for binary tree

 3  * public class TreeNode {

 4  *     int val;

 5  *     TreeNode left;

 6  *     TreeNode right;

 7  *     TreeNode(int x) { val = x; }

 8  * }

 9  */

10 public class Solution {

11     int maxValue(TreeNode r){

12         if(r == null) return Integer.MIN_VALUE;

13         int left = maxValue(r.left);

14         int right = maxValue(r.right);

15         return Math.max(r.val, Math.max(left, right));

16     }

17     int minValue(TreeNode r){

18         if(r == null) return Integer.MAX_VALUE;

19         int left = minValue(r.left);

20         int right = minValue(r.right);

21         return Math.min(r.val, Math.min(left, right));

22     }

23     public boolean isValidBST(TreeNode root) {

24         // Start typing your Java solution below

25         // DO NOT write main() function

26         if(root == null) 

27             return true;

28         if(maxValue(root.left) >= root.val || minValue(root.right) <= root.val)

29             return false;

30         return (isValidBST(root.left) && isValidBST(root.right));

31     }

32 }

 

Solution 2:

 1 public class Solution {

 2     public boolean validate(TreeNode root, int min, int max){

 3         if(root == null)

 4             return true;

 5         if(root.val > min && root.val < max && 

 6             validate(root.left, min, root.val) &&

 7             validate(root.right, root.val, max))

 8             return true;

 9         else

10             return false;

11     }

12     public boolean isValidBST(TreeNode root) {

13         // Start typing your Java solution below

14         // DO NOT write main() function

15         return validate(root, Integer.MIN_VALUE, Integer.MAX_VALUE);

16     }

17 }