题解 | #加起来和为目标值的组合(三)#

import java.util.*;


public class Solution {

    private List<List<Integer>> res = new ArrayList<>();

    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param k int整型 
     * @param n int整型 
     * @return int整型二维数组
     */
    public int[][] combination (int k, int n) {
        // write code here
        if (n < 1 || k < 1) {
            return new int[0][0];
        }
        backtrack(k, 1, n, new ArrayList<>());
        int[][] matrix = new int[res.size()][k];
        for (int i = 0; i < matrix.length; i++) {
            List<Integer> list = res.get(i);
            for (int j = 0; j < list.size(); j++) {
                matrix[i][j] = list.get(j);
            }
        }
        return matrix;
    }

    private void backtrack(int k, int start, int target, List<Integer> list) {
        if (list.size() == k && target == 0) {
            res.add(new ArrayList<>(list));
            return;
        }
        for (int i = start; i <= 9; i++) {
            if (target - i < 0) {
                break;
            }
            target -= i;
            list.add(i);
            backtrack(k, i + 1, target, list);
            target += i;
            list.remove(list.size() - 1);
        }
    }
}