存在重复元素

class Solution {
public:
    bool containsDuplicate(vector<int>& nums) 
    {
        sort(nums.begin(), nums.end());
        for (int i = 0; i < nums.size()-1; i++)
        {
            if (nums[i] == nums[i + 1])
                return true;
        }
        return false;
    }
};

class Solution {
public:
    bool containsDuplicate(vector<int>& nums) 
    {
        unordered_set<int> s;
        for (int x : nums) {
            //如果是重复元素，插入失败，返回尾迭代器
            if (s.find(x) != s.end()) {
                return true;
            }
            s.insert(x);
        }
        return false;
    }
};