217. Contains Duplicate

O(n) time O(n) space
朴素解法即可

class Solution {
public:
    bool containsDuplicate(vector<int>& nums) {
        unordered_set<int> s;
        for (int x : nums) {
            if (s.count(x)) return true;
            s.insert(x);
        }
        return false;
    }
};