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

return4.

tag: binary tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private class MyTree{
        int val;

        public MyTree(int val){
            this.val = val;
        }
    }

    int ans = 0;
    public int countUnivalSubtrees(TreeNode root) {
        if (root == null) return 0;

        dfs(root);

        return ans;
    }

    private int dfs(TreeNode root){
        if (root == null) return Integer.MAX_VALUE;
        if (root.left == null && root.right == null){
            ans++;
            return root.val;
        } 

        int leftRes = dfs(root.left);
        int rightRes = dfs(root.right);

        if ( (root.left == null || root.val == leftRes) && (root.right == null || root.val == rightRes) ){
            ans++;
            return root.val;
        }
        else{
            return Integer.MAX_VALUE;
        }
    }
}