965. Univalued Binary Tree

Question:

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:

  • Method 1: dfs ```Java /**
    • Definition for a binary tree node.
    • public class TreeNode {
    • int val;
    • TreeNode left;
    • TreeNode right;
    • TreeNode(int x) { val = x; }
    • } */ class Solution { private int pre; public boolean isUnivalTree(TreeNode root) { pre = root.val; return check(root.left) && check(root.right); } private boolean check(TreeNode node){ if(node == null) return true; else if(pre != node.val) return false; else{ return check(node.left) && check(node.right); } } } ```