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].

EXPLANATION:

其实这个题目一看的话就很容易了。就是在考察二叉树的遍历。但是需要记住的是，判断条件是不同的层级的节点，如果你仅仅是判断左右节点是否相同，那么很容易漏掉节点。

采用递归的方式就可以。

SOLUTION:

class Solution {
    public boolean isUnivalTree(TreeNode root) {
        if(root == null) return false;
        return isUnivalTreeHelper(root.left,root.val)&&isUnivalTreeHelper(root.right,root.val);
    }
    
    public static boolean isUnivalTreeHelper(TreeNode root,int pre){
        if(root == null) return true;
        if(root.val!=pre) return false;
        return isUnivalTreeHelper(root.left,root.val)&&isUnivalTreeHelper(root.right,root.val);
    }
}
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