【LeetCode】77. Combinations

问题描述

https://leetcode.com/problems/combinations/#/description

Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

For example,
If n = 4 and k = 2, a solution is:

[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

算法

类似全排列问题，使用回溯算法即可

代码

        public List<List<Integer>> combine(int n, int k) {            List<List<Integer>> res = new ArrayList<List<Integer>>();            fun(res, new ArrayList<Integer>(), 0, n, k);            return res;        }        public void fun(List<List<Integer>> res, List<Integer> list, int start, int n, int k) {            if(list.size() == k) {                res.add(new ArrayList<>(list));                return;            }            for(int i=start;i<n;i++) {                list.add(i+1);                fun(res, list, i+1, n, k);                list.remove(list.size()-1);            }        }