Unordered map in C++ not functioning as intended

Question

The program I am trying to make takes an "n * n" square matrix grid of size "n * n" with values in the range [1,n²]. Each integer appears exactly once except a number 'a' which appears twice and a number 'b' which is missing.

The task is to find the repeating and missing numbers 'a' and 'b'.

It should return a vector of two elements, {a,b}.

i.e. for the matrix {[1,2],[2,3]}, it should return vector {2,4}.

In my program, I am almost certain it logically outputs 'a' and 'b'. However, it always outputs [a,0]. 'b' is always 0.

Element 'b' should never be in the map, and hence the find function should determine the value of 'b'.

vector<int> findMissingAndRepeatedValues(vector<vector<int>>& grid) { 
 
    int size = grid.size(); 
    unordered_map<int,int> map; 
    for(int r = 0; r < size; r++) { 
        for(int c = 0; c < size; c++) { 
            map[grid[r][c]]++; 
        } 
    } 
    int a = 0; 
    int b = 0; 
    for(int i = 1; i < size*size + 1; i++) { 
        if(map[i] == 2) { 
            a = i; 
        } 
        else if(map.find(i) == map.end()) { 
            b = i; 
        } 
    } 
    return vector<int> {a,b}; 
} 

The map is being added created correctly. I believe it's something to do with the second for-loop.