【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);
	    	}
	    }