Leetcode 77: combination

link
subject ：

Ideas ： to flash back （ No duplicate elements cannot be checked ）

Similar to subsets , No duplication of data , And （2,1） and （1,2） It's a result , Therefore, you cannot check ;
Every node in the subset problem is a subset , But when the number of elements reaches k Only then path Put in res in ！

Be careful ：
In the set problem , It's an array ,for from 0 To n-1 ; The combination problem gives n,for from 1 To n ;

Java Realization ：

class Solution {
    
    List<List<Integer>> res=new LinkedList<>();
    LinkedList<Integer> path=new LinkedList<>();
    
    public List<List<Integer>> combine(int n, int k) {
    
        dfs(n,k,1); //  Be careful start from 1 Start 
        return res;
    }
    void dfs(int n,int k,int start){
    
       // Termination conditions , When the number of elements is k Then add 
        if(path.size()==k){
    
            res.add(new LinkedList(path));
        }
        for(int i=start;i<=n;i++){
    
            // Make a choice before recursion 
            path.add(i);
            // recursive 
            dfs(n,k,i+1);
            // Undo selection after recursion 
            path.removeLast();
        }
    }
}