[Leetcode] [Java] Combinations

Title:

Given-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],  [up],  [1,3],  [1,4],]

Test Instructions:

Given two integers n and K, return 1 ... all combinations of k digits in N.

Say

If N=4 and k=2, the solution is:

[  [2,4],  [3,4], [  2,3],  [up],  [1,3],  [1,4],]

Algorithm Analysis:

Recursive processing of sub-problems with a loop.

AC Code:

public class Solution {public    arraylist<arraylist<integer>> combine (int n, int k)    {         ArrayList <ArrayList<Integer>> res = new arraylist<arraylist<integer>> ();        if (n<=0 | | n<k)          return res;         Helper (N,k,1,new arraylist<integer> (), res);        return res;    }    private void Helper (int n, int k, int start, arraylist<integer> item, arraylist<arraylist<integer>> Res    {         if (item.size () ==k)         {             res.add (new arraylist<integer> (item));            return;         }         for (int i=start;i<=n;i++)//try each possibility number in the current position         {             item.add (i);             Helper (n,k,i+1,item,res); After selecting number for current position, process next position              Item.remove (Item.size ()-1);//Clear the Curren T position to try next possible number          }}}     

Copyright NOTICE: This article is the original article of Bo Master, reprint annotated source

[Leetcode] [Java] Combinations