map：自定义排序，初始化，以及key的替换

	struct mykey {
		int k;
		mykey(int a) {
			k=a;
		}
		bool operator<(const mykey& a)const  {
			return this->k>a.k;
		}
	};
	template<typename t>
	void replace_key(t& a,const typename t::key_type& oldk,
	                 const typename t::key_type& newk) {
		typename t::iterator pos=a.find(oldk);
		typename t::value_type temp(newk,pos->second);
        //不知道为啥定义这个temp，必须要初始化一下
		a.erase(pos);
		a.insert(typename t::value_type(newk,pos->second));
	}
	using p=pair<mykey,string>;
	int main(int argv,char** argc) {
	//	std::ios::sync_with_stdio(false);
	//	cin.tie(0);
	//	cout.tie(0);
		map<mykey,string,less<mykey>>dase {
			{
				3,"fsa"
			},
			{
				4,"Dfas"
			},
			{
				1,"o3r"
			}
		};
		using ty=decltype(dase)::value_type;
		replace_key(dase,3,100);
		for(const auto ele:dase) {
			cout<<ele.first.k<<" ";
		}
		return 0;
	}