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], ]
https://oj.leetcode.com/problems/combinations/
思路：递归，依次按要求填入结果。
public class Solution { public ArrayList<ArrayList<Integer>> combine(int n, int k) { ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>(); ArrayList<Integer> tmp = new ArrayList<Integer>(); combineHelper(res, tmp, 1, n, k); return res; } private void combineHelper(ArrayList<ArrayList<Integer>> res, ArrayList<Integer> tmp, int start, int n, int k) { if (k == 0) { res.add(new ArrayList<Integer>(tmp)); return; } for (int i = start; i <= n - k + 1; i++) { tmp.add(i); combineHelper(res, tmp, i + 1, n, k - 1); tmp.remove(tmp.size() - 1); } } public static void main(String[] args) { System.out.println(new Solution().combine(4, 2)); } }
[leetcode] Combinations,布布扣,bubuko.com
原文：http://www.cnblogs.com/jdflyfly/p/3815282.html
