【LeetCode】77. Combinations

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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);
            }
        }