The Problem:
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], ]
Notes:
- In each iteration, choose one number larger than the tail element , and append to the list.
Java Solution
public class Solution { public List<List<Integer>> combine(int n, int k) { List<List<Integer>> result = new ArrayList<List<Integer>>(); if(k==0) return result; for(int i = 1; i <= n-k+1; i++){ List<Integer> list = new ArrayList<Integer>(); list.add(i); result.add(list); } k--; while(k>0){ List<List<Integer>> temp = new ArrayList<List<Integer>>(); for(List<Integer> onelist : result){ int last = onelist.get(onelist.size()-1); for(int j = last+1; j<=n; j++){ List<Integer> newone = new ArrayList<Integer>(onelist); newone.add(j); temp.add(newone); } } result = temp; k--; } return result; } }
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