LeetCode_77 Combinations

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Link to original problem:
这里写链接内容

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],
]

Related problems:
39 Combination Sum: 这里写链接内容
46 Permutations: 这里写链接内容

这一题再将cur加入res的时机处理上与全遍历到结尾（一般是数组length处）再将cur加入res不同。而是即时检测，在进入数组length或者不满足继续放入物品的子函数之前，就放入res。避免了多次进入无效的子函数并多次将cur放入res。以下为代码：

public class Solution {    public List<List<Integer>> combine(int n, int k) {        List<List<Integer>> res = new ArrayList<List<Integer>>();        if(n <= 0 || k <= 0) return res;        List<Integer> cur = new ArrayList<Integer>();        helper(res, cur, k, 0, n);        return res;    }    private void helper(List<List<Integer>> res, List<Integer> cur, int remain, int start, int n){        if(remain == 0) return;        for(int ii = start; ii < n; ii++){            cur.add(ii+1);            if(remain == 1) res.add(new ArrayList<Integer>(cur));            helper(res, cur, remain-1, ii+1, n);            cur.remove(cur.size()-1);        }    }}

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