Combinations

Leetcode 77.

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class Solution {
    public List<List<Integer>> combine(int n, int k) {
       int[] nums = new int[n];
       for(int i = 1;i <= n;i++){
         nums[i -1] = i;
       }

       List<List<Integer>> ans = new ArrayList<>();
       List<Integer> list = new ArrayList<>(); 
       backtricking(nums,k,ans,list,0);
       return ans;
    }
    public void backtricking(int[] nums,int k,List<List<Integer>> ans, List<Integer> list,int input){
      //recursion stop condition 
      if(list.size() == k){
         //collect the results
         ans.add(new ArrayList<>(list));      
         return;
      }

      //for loop for this layer 
      for(int i = input; i < nums.length; i++){
         list.add(nums[i]);
         //recursion 
         backtricking(nums,k,ans,list,i + 1);
         //undo the previous operations 
         list.remove(list.size() - 1);
      }

    }
}