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private int[][] data;

public NumMatrix(int[][] matrix) {
    data = matrix;
}

public int sumRegion(int row1, int col1, int row2, int col2) {
    int sum = 0;
    for (int r = row1; r <= row2; r++) {
        for (int c = col1; c <= col2; c++) {
            sum += data[r][c];
        }
    }
    return sum;
}

public class NumMatrix {
    
    private int[][] prefixSum;
    public NumMatrix(int[][] matrix) {
        if (matrix.length == 0 || matrix[0].length == 0) return;
        int m = matrix.length;
        int n = matrix[0].length;
        prefixSum = new int[m + 1][n + 1];
        for (int i = 1; i <= m; i++) {
            for (int j = 1; j <= n; j++) {
                prefixSum[i][j] = matrix[i - 1][j - 1] + (prefixSum[i - 1][j] + prefixSum[i][j - 1] - prefixSum[i - 1][j - 1]);
            }
        }
    }
    
    public 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);
 */