public class DayCode {    public static void main(String[] args) {        int[][] nums = new int[][]{{1,2},{1,2}};        int ans = new NumMatrix(nums).sumRegion(1,1,1,1);        System.out.println("ans is " + ans);    }}
/** * https://leetcode-cn.com/problems/range-sum-query-2d-immutable/submissions/ */class NumMatrix {    int[][] preSums;
    public NumMatrix(int[][] matrix) {        int m = matrix.length;        if (m > 0) {            int n = matrix[0].length;            preSums = new int[m + 1][n + 1];            for (int i = 0; i < m; i++) {                for (int j = 0; j < n; j++) {                    preSums[i + 1][j + 1] = preSums[i][j + 1] + preSums[i + 1][j] - preSums[i][j] + matrix[i][j];                }            }        }    }
    public int sumRegion(int row1, int col1, int row2, int col2) {        return preSums[row2 + 1][col2 + 1] - preSums[row1][col2 + 1] - preSums[row2 + 1][col1] + preSums[row1][col1];    }}