# 77. Combinations (Medium)

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

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

```Input: n = 4, k = 2
Output:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]
```

## Solutions

### 1.

``````class Solution {
public List<List<Integer>> combine(int n, int k) {
List<Integer> track = new ArrayList<>();

List<List<Integer>> ans = new ArrayList<>();

getNext(0, n, k, track, ans);

return ans;
}

private void getNext(int depth, int n, int k, List<Integer> track, List<List<Integer>> ans) {
if (k == track.size()) {
// create a new copy, do not refer to it
List<Integer> solution = new ArrayList<>();
for (int x : track) {
solution.add(x);
}

ans.add(solution);

return;
}

for (int j = depth + 1; j <= n; j++) {
track.add(j);
getNext(j, n, k, track, ans);
track.remove(track.size()-1);
}
}
}
``````

### 2.

``````class Solution2 {
public List<List<Integer>> combine(int n, int k) {
List<Integer> track = new ArrayList<>();

List<List<Integer>> ans = new ArrayList<>();

getNext(1, n, k, track, ans);

return ans;
}

private void getNext(int depth, int n, int k, List<Integer> track, List<List<Integer>> ans) {
if (track.size() == k) {
// create a new copy, do not refer to it
List<Integer> sol = new ArrayList<>(track);

ans.add(sol);

return;
}

// above code should not be place at the head of function, otherwise mistakes occurs
if (depth > n) {
return;
}

track.add(depth);
getNext(depth + 1, n, k, track, ans);
track.remove(track.size() - 1);

getNext(depth + 1, n, k, track, ans);
}
}
``````

## References

Copyright © iovi.com 2017 all right reserved，powered by GitbookLast Modification: 2019-12-03 11:01:18