Combinations

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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],
]

有关组合的问题，给定了一个限定条件，长度为k的组合。同样用回溯法，回溯的条件就是每个结果的长度为k。往前搜索的时候，每次都加1，知道遍历完所有的可能。代码如下：

public class Solution {
    public List> combine(int n, int k) {
        LinkedList list = new LinkedList();
        List> llist = new LinkedList>();
        if(n < 1) return llist;
        getCombine(1, n, k, list, llist);
        return llist;
    }
    public void getCombine(int start, int n, int k, LinkedList list, List> llist) {
        if(list.size() == k) {
            llist.add(new LinkedList(list));
            return;
        }
        for(int i = start; i <= n; i++) {
            list.add(i);
            getCombine(i + 1, n, k, list, llist);
            list.removeLast();
        }
    }
}