Univalued Binary Tree

A binary tree is univalued if every node in the tree has the same value.

Return true if and only if the given tree is univalued.

 

Example 1:

Input: [1,1,1,1,1,null,1]
Output: true
Example 2:

Input: [2,2,2,5,2]
Output: false
 

Note:

  1. The number of nodes in the given tree will be in the range [1, 100].
  2. Each node's value will be an integer in the range [0, 99].

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isUnivalTree(TreeNode root) {
        if (root == null) return true;
        int val = root.val;
        boolean left = helper(root.left, val);
        boolean right = helper(root.right, val);
        return left && right;
    }
    
    private boolean helper(TreeNode root, int val) {
        if (root == null) return true;
        if (root.val != val) return false;
        boolean left = helper(root.left, val);
        boolean right = helper(root.right, val);
        return left && right;
    }
}