[LeetCode][Java] Combinations
题目:
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],
]
题意:
给定两个整数 n 和 k,返回1 ... n中k个数字的全部的组合。
例如说。
假设n=4 and k=2,解为:
[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]
算法分析:
用一个循环递归处理子问题。
AC代码:
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 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 current position to try next possible number } } }