0304.cpp

class NumMatrix {
    int n, m;
    vector<vector<int>> dp;

  public:
    NumMatrix(vector<vector<int>> &matrix)
        : n(matrix.size()), m(matrix[0].size()), dp(vector<vector<int>>(n, vector<int>(m, 0))) {
        for (int i = 0, sum = 0; i < n; i++)
            sum = dp[i][0] = matrix[i][0] + sum;
        for (int i = 0, sum = 0; i < m; i++)
            sum = dp[0][i] = matrix[0][i] + sum;
        for (int i = 1; i < n; i++) {
            for (int j = 1; j < m; j++) {
                dp[i][j] = matrix[i][j] + dp[i - 1][j] + dp[i][j - 1] - dp[i - 1][j - 1];
            }
        }
    }

    int sumRegion(int x1, int y1, int x2, int y2) {
        int res = dp[x2][y2];
        if (x1 > 0) res -= dp[x1 - 1][y2];
        if (y1 > 0) res -= dp[x2][y1 - 1];
        if (x1 > 0 && y1 > 0) res += dp[x1 - 1][y1 - 1];
        return res;
    }
};