304.cpp

class NumMatrix {
public:
    vector<vector<int>> prefixSum;
    NumMatrix(vector<vector<int>>& matrix) {
        int m = matrix.size();
        int n = matrix[0].size();
        prefixSum.resize(m + 1, vector<int>(n + 1, 0));
        for (int i = 1; i <= m; ++i) prefixSum[i][1] = prefixSum[i - 1][1] + matrix[i - 1][0];
        for (int j = 2; j <= n; ++j) prefixSum[1][j] = prefixSum[1][j - 1] + matrix[0][j - 1];
        for (int i = 2; i <= m; ++i) {
            for (int j = 2; j <= n; ++j) {
                prefixSum[i][j] = prefixSum[i - 1][j] + prefixSum[i][j - 1] + matrix[i - 1][j - 1] - prefixSum[i - 1][j - 1];
            }
        }
    }
    
    int sumRegion(int row1, int col1, int row2, int col2) {
        return prefixSum[row2 + 1][col2 + 1] - prefixSum[row1][col2 + 1] - prefixSum[row2 + 1][col1] + prefixSum[row1][col1];
    }
};

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