題意：給一個集合，求d=a+b+c，d最大且a,b,c,d下標不能是同一個

解題思路

a+b=d-c

另外,可以OJ看下0ms大佬們的程式碼.

#include "pch.h"
#include <string>
#include<iostream>
#include<map>
#include<memory.h>
#include<vector>


namespace cc
{
	using std::cout;
	using std::endl;
	using std::cin;
	using std::map;
	using std::vector;
	class Add
	{

	public:
		int ai;
		int bi;
		int sum;
		Add() {};
		Add(int a, int b, int sum) :ai(a), bi(b) {
			this->sum = sum;
		}
	};

	constexpr int N = 1000;
	int a[N];
	int n;

	map<int, vector<Add>> addMaps;
	int ok;
	int d;

	int cmp(void const* v1, void const* v2)
	{
		int* k1 = (int*)v1;
		int* k2 = (int*)v2;
		return *k1 - *k2;
	}

	void dump()
	{
		for (int i = 0;i < n;i++)
		{
			cout << " " << a[i];
		}
		cout << endl;
	}


	void add()
	{
		using IT = map<int, vector<Add>>::iterator;
		for (int i = 0;i < n - 1;i++)
			for (int j = i + 1;j < n;j++)
			{
				//all a+b
				auto sum = a[i] + a[j];
				Add A(i, j, sum);
				IT it = addMaps.find(sum);
				if (it == addMaps.end())
				{
					vector<Add> v;
					v.push_back(A);
					addMaps[sum] = v;
				}
				else
				{
					it->second.push_back(A);
				}
			}

		//all d-c
		for (int i = n - 1;i >= 0 && ok == 0;i--)
		{
			for (int j = n - 1;j >= 0;j--)
			{
				if (i == j)
					continue;
				auto sub = a[i] - a[j];
				IT it = addMaps.find(sub);
				if (it == addMaps.end())
				{
					continue;
				}
				vector<Add> adds = addMaps[sub];
				for (int k = 0;k < adds.size();k++)
				{
					if (adds[k].ai != i && adds[k].ai != j && adds[k].bi != i && adds[k].bi != j)
					{
						ok = 1;
						d = a[i];
					}
				}
				if (ok)
					break;
			}
		}


	}

	void read()
	{
		memset(a, 0, sizeof(a));
		ok = 0;
		addMaps.clear();
		for (int i = 0;i < n;i++)
		{
			cin >> a[i];
		}
		//sort,cong xiao dao da
		qsort(a, n, sizeof(int), cmp);
		//dump();
	}

	void solve()
	{
		while (cin >> n && n)
		{
			read();
			add();
			if (ok)
				cout << d << endl;
			else
				cout << "no solution" << endl;
		}

	}




};


int main()
{

#ifndef ONLINE_JUDGE
	freopen("d://1.text", "r", stdin);
#endif // !ONLINE_JUDGE
	cc::solve();

	return 0;
}