int missingNumber(int A[], int N){
    //declaring a vector to store the frequency of each number
    vector<int> freq(N+1, 0);
    
    //counting freqency of each number
    for(int i=0; i<N-1; i++)
        freq[A[i]]++;
    
    int res = 0;
    //checking if any element has freqency equal to 0
    for(int i=1; i<freq.size(); i++){
        if(freq[i] == 0){
            res = i;
            break;
        }
    }
	    
    if(res == 0)
        res = N;
    
    return res;
}

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        unordered_map<int, int> map;
 
        for(auto x: nums)
            map[x]++;
 
        int res = 0;
        for(int i=0; i<=nums.size(); i++){
            if(map.find(i) == map.end()){
                res = i;
                break;
            }
        }
 
        return res;
    }
};

class Solution {
public:
    int missingNumber(vector<int>& nums) {
       set<int> set;
 
        for(auto x: nums)
            set.insert(x);
 
        int res = 0;
        for(int i=0; i<=nums.size(); i++){
            if(set.find(i) == set.end()){
                res = i;
                break;
            }
        }
 
        return res;
    }
};

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        //using gaussian summation formula
        int n = nums.size();
        int expected_sum = n*(n+1)/2;
 
        int actual_sum = 0;
        for(auto x: nums)
            actual_sum += x;
 
        return (expected_sum - actual_sum);
    }
};