77 Combinations

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

Given two integersnandk, return all possible combinations ofknumbers out of 1 ...n.

For example,
Ifn= 4 andk= 2, a solution is:

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

分析及代码

……被之前题目卡住了，非常无耻地水了这题算人头

public List<List<Integer>> combine(int n, int k) {
    List<List<Integer>> res = new ArrayList<>();
    generate(res, new ArrayList<>(), n, k, 1);
    return res;
}

private void generate(List<List<Integer>> res, List<Integer> item, int n, int k, int index) {
    if(item.size() == k) {
        res.add(new ArrayList<>(item));
        return;
    }
    for(int i = index; i <= n; i++) {
        item.add(i);
        generate(res, item, n, k, i + 1);
        item.remove(item.size() - 1);
    }
}