LeetCode-Univalued Binary Tree

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Description：
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:

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

题意：判断一颗二叉树的所有节点值是否都相同；

解法：我们只需要遍历一遍二叉树，判断所有的节点值与根节点是否相同即可；

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int preVal = -1;
    public boolean isUnivalTree(TreeNode root) {
        if (root == null) {
            return true;
        }
        if (preVal == -1) {
            preVal = root.val;
        }
        return preVal == root.val && isUnivalTree(root.left) &&
            isUnivalTree(root.right);
    }
}