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.

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
      public int countUnivalSubtrees(TreeNode root) {
        if (root == null) return 0;
        int[] result = helper(root);
        return result[0] + result[1];
    }

    private int[] helper(TreeNode root) {
        if (root == null) return new int[2];

        int[] leftArray = helper(root.left);
        int[] rightArray = helper(root.right);

        int[] result = new int[2];
        if (root.left != null && root.right != null) {
            if (leftArray[0] == 1 && rightArray[0] == 1 && root.val == root.left.val && root.val == root.right.val) {
                result[0] = 1;
            }
        } else if (root.left != null) {
            if (leftArray[0] == 1 && root.val == root.left.val) {
                result[0] = 1;
            }
        } else if (root.right != null) {
            if (rightArray[0] == 1 && root.val == root.right.val) {
                result[0] = 1;
            }
        } else {
            result[0] = 1;
        }

        result[1] = leftArray[1] + rightArray[1] + leftArray[0] + rightArray[0];
        return result;
    }
}