class Solution {
public:
int getMissingNumber(vector<int>& nums) {
//不用判空,如果传入的是空数组,那么哈希表S也是空的
unordered_set<int> S;
for (int i = 0; i <= nums.size(); i ++ ) S.insert(i);
for (int x : nums) S.erase(x);
return *S.begin();
}
};