Leetcode: Count Univalue Subtrees

Given a binary tree, count the number of uni-value subtrees.

A Uni-value subtree means all nodes of the subtree have the same value.

For example:
Given binary tree,

              5
             / \
            1   5
           / \   \
          5   5   5

return 4.

Understand the problem:
The problem can be solved by divide-and-conquer. The only trick is to use different return values to mark different cases. 
Integer.MIN_VALUE -- Mark the subtree is not univaled. 
Integer.MAX_VALUE -- Mark if the root is null. 

Code (Java):

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/**  * Definition for a binary tree node.  * public class TreeNode {  *     int val;  *     TreeNode left;  *     TreeNode right;  *     TreeNode(int x) { val = x; }  * }  */ public class Solution {     private int count = 0;     public int countUnivalSubtrees(TreeNode root) {         if (root == null) {             return 0;         }                   countUnivalSubtreesHelper(root);                   return count;     }           private int countUnivalSubtreesHelper(TreeNode root) {         if (root == null) {             return Integer.MAX_VALUE;         }                   // Divide         int left = countUnivalSubtreesHelper(root.left);         int right = countUnivalSubtreesHelper(root.right);                   // left and right are all empty         if (left == Integer.MAX_VALUE && right == Integer.MAX_VALUE) {             count++;             return root.val;         } else if (left == Integer.MAX_VALUE || right == Integer.MAX_VALUE) {             if (root.val == left || root.val == right) {                 count++;                 return root.val;             } else {                 return Integer.MIN_VALUE;             }         } else {             if (root.val == left && root.val == right) {                 count++;                 return root.val;             } else {                 return Integer.MIN_VALUE;             }         }     } }