Validate Binary Search Tree - LeetCode

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:

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
int maxValue(TreeNode r){
if(r == null) return Integer.MIN_VALUE;
int left = maxValue(r.left);
int right = maxValue(r.right);
return Math.max(r.val, Math.max(left, right));
}
int minValue(TreeNode r){
if(r == null) return Integer.MAX_VALUE;
int left = minValue(r.left);
int right = minValue(r.right);
return Math.min(r.val, Math.min(left, right));
}
public boolean isValidBST(TreeNode root) {
// Start typing your Java solution below
// DO NOT write main() function
if(root == null)
return true;
if(maxValue(root.left) >= root.val || minValue(root.right) <= root.val)
return false;
return (isValidBST(root.left) && isValidBST(root.right));
}
}

Solution 2:

public class Solution {
public boolean validate(TreeNode root, int min, int max){
if(root == null)
return true;
if(root.val > min && root.val < max &&
validate(root.left, min, root.val) &&
validate(root.right, root.val, max))
return true;
else
return false;
}
public boolean isValidBST(TreeNode root) {
// Start typing your Java solution below
// DO NOT write main() function
return validate(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
}
}