leetcode 77 组合

package com.example.lettcode.dailyexercises;

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

/**
 * @Class Combine
 * @Description 77 组合
 * 给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。
 * <p>
 * 示例:
 * 输入: n = 4, k = 2
 * 输出:
 * [
 * [2,4],
 * [3,4],
 * [2,3],
 * [1,2],
 * [1,3],
 * [1,4],
 * ]
 * @Author
 * @Date 2020/9/11
 **/
public class Combine {
    public static List<List<Integer>> combine(int n, int k) {
        if (k > n) return new ArrayList<>();
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> integers = new ArrayList<>();
        recur(res, integers, n, k, 1);
        return res;
    }

    // 回溯+递归
    public static void recur(List<List<Integer>> res, List<Integer> integers, int n, int k, int idx) {
        if (integers.size() == k) {
            res.add(new ArrayList<>(integers));
            return;
        }
        if (idx > n) {
            return;
        }
        for (int i = idx; i <= n; i++) {
            integers.add(i);
            recur(res, integers, n, k, i + 1);
            integers.remove(integers.size() - 1);
        }
    }


    public static void main(String[] args) {
        int n = 4;
        int k = 2;
        List<List<Integer>> res = combine(n, k);
        System.out.println("Combine demo01 result:");
        for (List<Integer> integerList : res) {
            System.out.println();
            for (Integer integer : integerList) {
                System.out.print("," + integer);
            }
        }
    }
}