LeetCode:Combinations

Combinations

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],
]

» Solve this problem

Use iterations to generate different levels of solutions. 
The first level solution is the set of all first numbers in the final solution is {1,2,...,n-k+1}. To avoid duplicates, for the next level, always add a larger number to the element in current level. 

public class Solution {
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if(k==0) return res;
        res.add(new ArrayList<Integer>());
        for(int i=0;i<k;i++){
             ArrayList<ArrayList<Integer>> prev = new ArrayList<ArrayList<Integer>>();
             for(ArrayList<Integer> temp:res){
                int a=0;
                if(temp.size()>0)
                    a=temp.get(temp.size()-1);
                for(int j=a+1;j<=n-k+1+i;j++){
                    ArrayList<Integer> b= new ArrayList<Integer>(temp);
                    b.add(j);
                    prev.add(b);
                }
             }
            res = new ArrayList<ArrayList<Integer>>(prev);
        }
        return res;
    }
}