965. Univalued Binary Tree.md

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

  /**
   * 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);
          }
      }
  }