Leetcode 77. Combinations

Description:

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],
]

Thinking:

Combination problems means we should consider the duplicate since combination doesn't care about the order.So we need to use a int pos as a parameter in the helper function.

Complexity:

The time complexity is approximately n!.

Steps:

1. check whether input is valid.
2. create the helper function and decide which parameters we need and the condition that we can return

   Code:
   

   public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        if(n < k) return new ArrayList<List<Integer>>();
        List<List<Integer>> result = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        combineHelper(result, list, n, k, 1);
        return result;
    }
    public void combineHelper(List<List<Integer>> result, List<Integer> list, int n, int k, int pos){
        if(list.size() == k){
            result.add(new ArrayList<Integer>(list));
            return;
        }
        if(pos > n){
            return;
        }
        for(int i = pos; i <= n; i++){
            list.add(i);
            combineHelper(result, list, n, k, i+1);
            list.remove(list.size()-1);
        }
    }
   }
   

   Conclusion:
   

   This can be a template for the problem of combination.