A nice two pointer problem

Rotate a Box Under Gravity

We can solve the problem by using two pointers. For each row, we use two pointers:

• left and right
• left indicate the box we shoud move
• right indicate the available location to be moved.

Code

vector<vector<string>> rotate(vector<vector<string>>& matrix){
    const int n = matrix.size(), m = matrix[0].size();
    vector<vector<string>> newmatrix(m, vector<string>(n));
    for (int i = 0; i < n; ++i){
        for (int j = 0; j < m; ++j){
            newmatrix[j][n-i-1] = matrix[i][j];
        }    
    }
    return newmatrix;
}
vector<vector<string>> solve(vector<vector<string>>& matrix) {
    if (matrix.empty() || matrix[0].empty()) return matrix;
    const int n = matrix.size(), m = matrix[0].size();
    for (int i = 0; i < n; ++i){
        int l = m -1, r = m -1;
        while (l >= 0){
            if (matrix[i][l] == "*"){
                l = r - 1;
                r = r - 1;
            } else if (matrix[i][l] == "."){
                l--;
            } else {
                swap(matrix[i][l], matrix[i][r]);
                l--;r--;
            }
        }
    }
    return rotate(matrix);
    
}