class NumMatrix {
public:
    vector<vector<int>> mat;
    NumMatrix(vector<vector<int>>& matrix) {
        mat = matrix;
        int m = matrix.size(), n = matrix[0].size();
        for (int i = 0; i < m; i++)
            for (int j = 1; j < n; j++)
                mat[i][j] = matrix[i][j] + mat[i][j-1];
    }
    
    int sumRegion(int row1, int col1, int row2, int col2) {
        int ret = 0;
        for (int i = row1; i <= row2; i++)
            if (col1 == 0) ret += mat[i][col2];
            else ret += mat[i][col2] - mat[i][col1-1];
        return ret;
    }
};

/**
 * Your NumMatrix object will be instantiated and called as such:
 * NumMatrix* obj = new NumMatrix(matrix);
 * int param_1 = obj->sumRegion(row1,col1,row2,col2);
 */