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].

recursion

time: O(n), space: O(height)

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