304. Range Sum Query 2D Immutable

leetcode - 304. Range Sum Query 2D - Immutable

Submission

class NumMatrix {
    private final int[][] prefixRow;

    public NumMatrix(int[][] matrix) {
        int n = matrix.length;
        int m = matrix[0].length;
        this.prefixRow = new int[n][m + 1];
        for (int i = 0; i < n; i++) {
            for (int j = 1; j < m + 1; j++) {
                prefixRow[i][j] = prefixRow[i][j - 1] + matrix[i][j - 1];
            }
        }
    }

    public int sumRegion(int row1, int col1, int row2, int col2) {
        int sum = 0;
        for (int i = row1; i <= row2; i++) {
            sum += prefixRow[i][col2 + 1] - prefixRow[i][col1];
        }
        return sum;
    }
}

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