Answer key
They’re very clear, so do what they say, flip it horizontally first, and then flip it backwards.
Horizontal reverse can use its own reverse, reverse directly and 1 XOR can be.
code

class Solution {
public:
    vector<vector<int>> flipAndInvertImage(vector<vector<int>>& A) {
        int n = A.size();
        for(int i = 0; i < n; i++){
            reverse(A[i].begin(),A[i].end());
        }
        for(int i = 0; i < n; i++){
            for(int j = 0; j < A[0].size(); j++){
                A[i][j] = A[i][j]^1;
            }
        }
        return A;
    }
};