C++

1class Solution {
2  public:
3    set<int> s;
4    bool isUnivalTree(TreeNode* root) {
5      preorder(root);
6      return s.size() == 1;
7    }
8
9    void preorder(TreeNode* node) {
10      if(!node || s.size() > 1) return;
11      s.insert(node->val);
12      preorder(node->left);
13      preorder(node->right);
14    }
15};