题解 | #组合#

import java.util.*;


public class Solution {

    private ArrayList<ArrayList<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 n int整型 
     * @param k int整型 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> combine (int n, int k) {
        // write code here
        backtrack(n, k, 1, new ArrayList<>());
        return res;
    }

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