Combinations

77. Combinations

Difficulty: Medium

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],
]

Solution

Language: Java

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new LinkedList<>();
        
        dfs(n, k, 1, new LinkedList<Integer>(), result);
        return result;
    }
    
    public void dfs(int n, int k, int pos, LinkedList<Integer>path, List<List<Integer>>result){
        if(path.size() == k){
            result.add(new LinkedList<Integer>(path));
            return;
        }
        
        for(int i=pos;i<=n;i++){
            path.addLast(i);
            dfs(n, k, i+1, path, result);
            path.removeLast();
        }
    }
}