Count Univalue Subtrees leetcode

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.

就是对一个树找子树值和root相同的个数, 对于例子中 三个叶节点5都算, 另外一个是root的右儿子 因为它left child为空 right child值跟本身相等

public class Solution {
    public int countUnivalSubtrees(TreeNode root) {
        int[] res = new int[1];
        if (root == null) {
            return 0;
        }
        helper(root, res);
        return res[0];
    }
    public boolean helper(TreeNode root, int[] res) {
        if (root.left == null && root.right == null) {
            res[0]++;
            return true;
        } else if (root.left == null && root.right != null) {
            if (helper(root.right, res) && root.val == root.right.val) {
                res[0]++;
                return true;
            } else {
                return false;
            }
        } else if (root.right == null && root.left != null) {
            if (helper(root.left, res) && root.left.val == root.val) {
                res[0]++;
                return true;
            } else {
                return false;
            }
        } else {
            boolean l = helper(root.left, res);//必须单独列出来因为如果root.left = false 仍要递归root.right以对res进行操作
            boolean r = helper(root.right, res);
            if (l && r && root.val == root.left.val && root.val == root.right.val) {
                res[0]++;
                return true;
            } else {
                return false;
            }
        }
    }
}