https://leetcode.com/problems/missing-number/description/
class Solution {
public:
int missingNumber(vector<int>& nums) {
set<int>st;
for(int i=0; i<nums.size(); i++) st.insert(nums[i]);
for(int i=0;i<=nums.size();i++){
if(!st.count(i)) return i;
}
}
};