Strassen's Matrix Multiplication

// Strassen skeleton (square matrices, power-of-two size)
vector<vector<int>> addM(const vector<vector<int>>& A, const vector<vector<int>>& B){
    int n=A.size(); vector<vector<int>> C(n, vector<int>(n));
    for(int i=0;i<n;i++) for(int j=0;j<n;j++) C[i][j]=A[i][j]+B[i][j];
    return C;
}
vector<vector<int>> subM(const vector<vector<int>>& A, const vector<vector<int>>& B){
    int n=A.size(); vector<vector<int>> C(n, vector<int>(n));
    for(int i=0;i<n;i++) for(int j=0;j<n;j++) C[i][j]=A[i][j]-B[i][j];
    return C;
}