leetcode Combinations

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Combinations 原题地址：

https://oj.leetcode.com/problems/combinations/

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],]

public class Solution {    private List<List<Integer>> list = new ArrayList<List<Integer>>();public List<List<Integer>> combine(int n, int k) {if (k > n || k == 0)return list;LinkedList<Integer> _list = new LinkedList<Integer>();helpCombine(_list, n, 1, k);return list;}private void helpCombine(LinkedList<Integer> _list, int n, int idx, int k) {if (k == 0) {LinkedList<Integer> temp = (LinkedList<Integer>) _list.clone();list.add(temp);return;}if (idx > n)return;if (idx + k == n+1) {LinkedList<Integer> temp = (LinkedList<Integer>) _list.clone();for (int i = idx; i <= n; i++) {temp.add(i);}list.add(temp);return;}_list.add(idx);helpCombine(_list, n, idx+1, k-1);_list.pollLast();helpCombine(_list, n, idx+1, k);}}

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