Leetcode_77 组合问题

题目描述

题解

java

import java.util.*;
public class Leetcode_77 {
    public List> combine(int n, int k) {
        List> ret = new ArrayList>();
        if(n<=0||k<=0||k>n)
            return ret;
        ArrayList p = new ArrayList();
        findCombine(n,k,1,p,ret);
        return ret;

    }
    private void findCombine(int n, int k, int start, List c,List> ret)
    {

        if(c.size() == k)
        {
            ret.add(new ArrayList<>(c));
           // System.out.println(c.toString());
            return;
        }
        for(int i = start; i<=n; i++)
        {
            c.add(i);

            findCombine(n,k,i+1,c, ret);
            //这边我写成start+1，调了半天怎么也不对
            System.out.println(c.toString());
            c.remove(c.size()-1);
        }

    }


    public static void main(String[] args)
    {
        Leetcode_77 a = new Leetcode_77();
        List> ret = a.combine(4,2);
        System.out.print(ret.toString());
    }
}

cPP

class Solution {
private:
    vector<vector<int>> res;
    void generateCombinations(int n, int k, int start, vector<int> &c)
    {
        if(c.size() == k)
        {
            res.push_back(c);
            return;
        }
        for(int i = start;i<=n;i++){
            c.push_back(i);
            generateCombinations(n,k,i+1,c);
            c.pop_back();
        }
        return;

    }
public:
    vector<vector<int>> combine(int n, int k) {
        res.clear();
        if(n <= 0 || k<=0 || k>n)
            return res;

        vector<int> c;
        generateCombinations(n,k,1,c);
        return res;

    }
};

代码优化

import java.util.*;
public class Leetcode_77 {
    public List> combine(int n, int k) {
        List> ret = new ArrayList>();
        if(n<=0||k<=0||k>n)
            return ret;
        ArrayList p = new ArrayList();
        findCombine(n,k,1,p,ret);
        return ret;

    }
    private void findCombine(int n, int k, int start, List c,List> ret)
    {

        if(c.size() == k)
        {
            ret.add(new ArrayList<>(c));
           // System.out.println(c.toString());
            return;
        }
        for(int i = start; i<=n-(k-c.size())+1; i++)
        {
            c.add(i);

            findCombine(n,k,i+1,c, ret);
            System.out.println(c.toString());
            c.remove(c.size()-1);
        }

    }


    public static void main(String[] args)
    {
        Leetcode_77 a = new Leetcode_77();
        List> ret = a.combine(10,8);
        System.out.print(ret.toString());
    }
}