[leetcode] 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],]
https://oj.leetcode.com/problems/combinations/

思路：遞迴，依次按要求填入結果。

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));    }}

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