package LeetCode;

import java.util.ArrayList;
import java.util.List;

public class LC_77 {
    static ArrayList<Integer> path = new ArrayList<>();
    static List<List<Integer>> res = new ArrayList<>();

    public static void main(String[] args) {
        combine(5, 4);
        System.out.println(res);
    }


    public static List<List<Integer>> combine(int n, int k) {
        backtracing(n, k, 1);
        return res;
    }

    public static void backtracing(int n, int k, int start) {
        if (path.size() == k) {
            //res.add(path);  //这样是将path的引用添加到了res中，是不对的，后续对path的修改会影响res的值
            res.add(new ArrayList<>(path));
            return;
        }
        for (int i = start; i <= n; i++) {
            path.add(i);
            backtracing(n, k, i + 1);
            path.remove(path.size() - 1);
        }
    }
}