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],]

 

 

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