Range sum query - Immutable

Problem

TC: O(n*m) for creating the prefix[][] sum matrix, O(row1 row2) for calculating the sum of the given rectangle
SC: O(n*m) for using the prefix[][] sum matrix

class NumMatrix {
    int prefix[][];
    public NumMatrix(int[][] matrix) {
        prefix = new int[matrix.length][matrix[0].length];
        for(int i=0;i<matrix.length;i++){
            int current = 0;
            for(int j = 0;j<matrix[0].length;j++){
                current+=matrix[i][j];
                prefix[i][j] = current;
            }
        }
    }

    public int sumRegion(int row1, int col1, int row2, int col2) {
        int sum =0;
        for(int i =row1;i<=row2;i++){
            int right = prefix[i][col2];
            int left = (col1>0) ? prefix[i][col1-1] : 0;
            sum+=right-left;
        }
        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);
 */

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