LeetCode-304-Range-Sum-Query-2D-Immutable.java

class NumMatrix {

    int[][] sum;
    
    public NumMatrix(int[][] matrix) {
        if (matrix != null && matrix.length > 0 && matrix[0].length > 0) {
            sum = new int[matrix.length][matrix[0].length];
        
            sum[0][0] = matrix[0][0];

            for (int i = 1; i < matrix.length; i++) {
                sum[i][0] = sum[i - 1][0] + matrix[i][0];
            }
            
            for (int j = 1; j < matrix[0].length; j++) {
                sum[0][j] = sum[0][j - 1] + matrix[0][j];
            }
            
            for (int i = 1; i < matrix.length; i++) {
                for (int j = 1; j < matrix[0].length; j++) {
                    sum[i][j] = sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1] + matrix[i][j];
                }
            }
        }
    }
    
    public int sumRegion(int row1, int col1, int row2, int col2) {
        if (row1 == 0 && col1 == 0) return sum[row2][col2];
        
        if (row1 == 0) return sum[row2][col2] - sum[row2][col1 - 1];
        
        if (col1 == 0) return sum[row2][col2] - sum[row1 - 1][col2];
        
        return sum[row2][col2] - sum[row1 - 1][col2] - sum[row2][col1 - 1] + sum[row1 - 1][col1 - 1];
    }
}

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