Range Sum Query 2D - Immutable

这道题一定要注意下标，一开始自己做的全都没有下标-1这一操作。

public class NumMatrix {    int[][] sum;    public NumMatrix(int[][] matrix) {        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {            return;        }        int row = matrix.length;        int col = matrix[0].length;        sum = new int[row][col];        for (int i = 0; i < row; i++) {            if (i == 0) {                sum[i][0] = matrix[0][0];            } else {                sum[i][0] = matrix[i][0] + sum[i - 1][0];            }        }        for (int i = 1; i < col; i++) {            sum[0][i] = matrix[0][i] + sum[0][i - 1];        }        for (int i = 1; i < row; i++) {            for (int j = 1; j < col; j++) {                sum[i][j] = matrix[i][j] + sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1];            }        }    }    public int sumRegion(int row1, int col1, int row2, int col2) {        if (sum == null) {            return 0;        }        if (row1 == 0 && col1 == 0) {            return sum[row2][col2];        } else if (row1 == 0) {            return sum[row2][col2] - sum[row2][col1 - 1];        } else if (col1 == 0) {            return sum[row2][col2] - sum[row1 - 1][col2];        } else {            return sum[row2][col2] - sum[row2][col1 - 1] - sum[row1 - 1][col2] + sum[row1 - 1][col1 - 1];        }    }}// Your NumMatrix object will be instantiated and called as such:// NumMatrix numMatrix = new NumMatrix(matrix);// numMatrix.sumRegion(0, 1, 2, 3);// numMatrix.sumRegion(1, 2, 3, 4);