四数之和为target特定值的序列

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一、算法思路是简单的

（1）先求出所有的两数之和，将和与下表存储到mutimap中；

（2）利用 val = { target-（1）两数和 } 去查询multimap ；

（3）如果查到，则记录下对应的序列；如果没有，则不用记录。

二、代码是复杂的

（1）有个四元组的operator< 重载问题，需要复杂的 if/else结构。原来设想的太简单的。

（2）需要multimap、set、vector的综合运用，才能完成复杂的逻辑。

（3）友元的使用还是第一次。

三、代码分为两部分，类以及main测试，如下：

（类）：

class node{//friend bool operator==(const node &l,const node &r);//friend bool operator!=(const node &l,const node &r);friend class solution;friend bool operator< (const node &l,const node &r);friend ostream& operator<<(ostream& out,const node &four);public:node():first(0),second(0),third(0),forth(0){}node(int a,int b,int c, int d):first(a),second(b),third(c),forth(d){}public:int first;int second;int third;int forth;};/*bool operator==(const node &l,const node &r){return (l.first==r.first) && (l.second==r.second) && (l.third==r.third) && (l.forth == r.forth);}bool operator!=(const node &l,const node &r){return !(l==r);}*/bool operator<(const node &l,const node &r){//return r.first>l.first?(r.second>l.second?(r.third>l.third?(r.forth>l.forth?1:0):0):0):0;if(l.first<r.first) return true;else if(l.first>r.first) return false;else{if(l.second<r.second) return true;else if (l.second>r.second) return false;else{if(l.third<r.third) return true;else if(l.third>r.third) return false;else{if(l.forth<r.forth) return true;else if(l.forth>r.forth) return false;else return false;}}}}ostream& operator<<(ostream& out,const node &four){out<<four.first<<" "<<four.second<<" "<<four.third<<" "<<four.forth;return out;}class Solution {public:    vector<vector<int> > fourSum(vector<int> &num, int target) {        vector<vector<int> > res;        vector<int> row;        if(num.size()<4) return res;        multimap<int,pair<int,int> > ht;         multimap<int,pair<int,int> >::iterator it,pos;        set<node> s;                int n = num.size();        for(int i =0 ; i< n-1; ++i)            for(int j =i+1 ; j<n;++j){                int tmp = num[i]+num[j];                //ht[tmp]=make_pair(i,j);                ht.insert(make_pair(tmp,make_pair(i,j)));            }                for(it=ht.begin();it!=ht.end();++it){            int k = (*it).first;            int key= target - k;            if(ht.count(key)!=0)                for(pos=ht.lower_bound(key);pos!=ht.upper_bound(key);++pos)                    if(                        (*it).second.first!= (*pos).second.first &&                        (*it).second.second!= (*pos).second.second &&                        (*it).second.second!= (*pos).second.first &&                        (*it).second.first!= (*pos).second.second                    ){//cout<<"test ";                    //cout<<(*it).second.first<<" "<<(*it).second.second<<" ";                    //cout<<(*pos).second.first<<" "<<(*pos).second.second<<endl;                    row.clear();                    row.push_back(num[(*it).second.first]);                    row.push_back(num[(*it).second.second]);                    row.push_back(num[(*pos).second.first]);                    row.push_back(num[(*pos).second.second]);                    sort(row.begin(),row.end());                    node x(row[0],row[1],row[2],row[3]);                    //cout<<x<<endl;                    //x.first=row[0];x.second=row[1];x.third=row[2];x.forth=row[3];                    //cout<<s.count(x)<<endl;                    if(s.find(x)==s.end()) {s.insert(x);res.push_back(row);}                }        }                return res;    }};

（测试）：

#include <iostream>#include <algorithm>#include <map>#include <set>#include <vector>using namespace std;class node{//friend bool operator==(const node &l,const node &r);//friend bool operator!=(const node &l,const node &r);friend bool operator< (const node &l,const node &r);friend ostream& operator<<(ostream& out,const node &four);public:node():first(0),second(0),third(0),forth(0){}node(int a,int b,int c, int d):first(a),second(b),third(c),forth(d){}public:int first;int second;int third;int forth;};/*bool operator==(const node &l,const node &r){return (l.first==r.first) && (l.second==r.second) && (l.third==r.third) && (l.forth == r.forth);}bool operator!=(const node &l,const node &r){return !(l==r);}*/bool operator<(const node &l,const node &r){//return r.first>l.first?(r.second>l.second?(r.third>l.third?(r.forth>l.forth?1:0):0):0):0;if(l.first<r.first) return true;else if(l.first>r.first) return false;else{if(l.second<r.second) return true;else if (l.second>r.second) return false;else{if(l.third<r.third) return true;else if(l.third>r.third) return false;else{if(l.forth<r.forth) return true;else if(l.forth>r.forth) return false;else return false;}}}}ostream& operator<<(ostream& out,const node &four){out<<four.first<<" "<<four.second<<" "<<four.third<<" "<<four.forth;return out;}vector<vector<int> > fourSum(vector<int> &num, int target) {        vector<vector<int> > res;        vector<int> row;        if(num.size()<4) return res;        multimap<int,pair<int,int> > ht;         multimap<int,pair<int,int> >::iterator it,pos;        set<node> s;                int n = num.size();        for(int i =0 ; i< n-1; ++i)            for(int j =i+1 ; j<n;++j){                int tmp = num[i]+num[j];                //ht[tmp]=make_pair(i,j);                ht.insert(make_pair(tmp,make_pair(i,j)));            }                for(it=ht.begin();it!=ht.end();++it){            int k = (*it).first;            int key= target - k;            if(ht.count(key)!=0)                for(pos=ht.lower_bound(key);pos!=ht.upper_bound(key);++pos)                    if(                        (*it).second.first!= (*pos).second.first &&                        (*it).second.second!= (*pos).second.second &&                        (*it).second.second!= (*pos).second.first &&                        (*it).second.first!= (*pos).second.second                    ){//cout<<"test ";                    //cout<<(*it).second.first<<" "<<(*it).second.second<<" ";                    //cout<<(*pos).second.first<<" "<<(*pos).second.second<<endl;                    row.clear();                    row.push_back(num[(*it).second.first]);                    row.push_back(num[(*it).second.second]);                    row.push_back(num[(*pos).second.first]);                    row.push_back(num[(*pos).second.second]);                    sort(row.begin(),row.end());                    node x(row[0],row[1],row[2],row[3]);                    //cout<<x<<endl;                    //x.first=row[0];x.second=row[1];x.third=row[2];x.forth=row[3];                    //cout<<s.count(x)<<endl;                    if(s.find(x)==s.end()) {s.insert(x);res.push_back(row);}                }        }                return res;    }int main() {// your code goes herevector<int> num;int n;int target;int tmp;cin>>n>>target;for(int i=0 ; i<n; i++){cin>>tmp;num.push_back(tmp);}vector<vector<int> > res;res = fourSum(num,target);if(res.size()==0){cout<<"empty result "<<endl; return 0;}int row = res.size();//if(!row) return 0;int col = res[0].size();//if(!col) return 0;for(int i=0 ; i< row; i++){for(int j =0 ; j< col; j++)cout<<res[i][j]<<" ";cout<<endl;}return 0;}

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