217. Contains Duplicate

Given an array of integers, find if the array contains any duplicates. Your function should return true if any value appears at least
twice in the array, and it should return false if every element is distinct.

用set，如果当前元素已经存在于set中，就说明有重复。

class Solution {
public:
bool containsDuplicate(vector<int>& nums) {
unordered_set<int> mySet;
for(int i = 0;i<nums.size();++i)
{
if(mySet.find(nums[i]) != mySet.end())
return true;
mySet.insert(nums[i]);
}

return false;
}
};