【codechef】Special Shortest Walk（STL妙用，优先队列）

1 ≤ Sum of N over all test cases ≤ 100000 
1 ≤ Sum of M over all test cases ≤ 500000 
3 ≤ N ≤ 100000   
2 ≤ M ≤ 500000   
1 ≤ T ≤ 2000 
1 ≤ Z ≤ 100000000  
1 ≤ X,Y ≤ N  
 X != Y 
source !=  sink and there are no multi edges.
source and sink will not be connected by a direct edge

Sample Input
3
5 6
4 2 8
1 4 6
2 3 10
3 1 10
1 2 3
3 5 3
4 3
4 3
1 4 4
2 4 5
2 3 6
4 3
5 5
1 2 100
2 3 80
3 4 90
4 2 85
2 5 120
1 5
    
Sample Output
19
No Solution
475

Explanation
For First Test Case: Shortest Valid Walk: 4->1->2->3  
For Second Test Case: There is no valid Walk satisfying the constraints.

点击打开链接

优先队列priority_queue 用法详解 优先队列是队列的一种，不过它可以按照自定义的一种方式（数据的优先级）来对队列中的数据进行动态的排序 每次的push和pop操作，队列都会动态的调整，以达到我们预期的方式来存储。 例如：我们常用的操作就是对数据排序，优先队列默认的是数据大的优先级高 所以我们无论按照什么顺序push一堆数，最终在队列里总是top出最大的元素。

#include <cstdio>
#include <algorithm>
#include <vector>
#include <queue>
using namespace std;
#define MAXN 100000
#define MAXM 500000
vector< pair<int, int> > L[MAXN],L2[MAXN];
struct node{
	int pos,parity,last;
	long long distance;
	
	node(){}
	node(int _pos, int _parity, int _last, long long _distance): 
		pos(_pos),parity(_parity),last(_last),distance(_distance){}

	bool operator < (node X) const{
		return distance > X.distance; //优先队列排序方式
	}
};

long long dijkstra(int source, int sink){
	priority_queue<node> Q;
	Q.push(node(source,0,0,0));
	
	while(!Q.empty()){
		node cur = Q.top();
		Q.pop();
		if(cur.pos == sink) return cur.distance;
		if(cur.parity == 1){  //第偶数个
			while(!L2[cur.pos].empty()){
				int w = L2[cur.pos].back().first;
				if(w < cur.last){ //下一个权值比当前这个小
					Q.push(node(L2[cur.pos].back().second,0,w,cur.distance + w)); //将L2里与这个点相连的点从小到大Push入
					L2[cur.pos].pop_back();
				} else break;
			}
		}
		else{  //第奇数个
			while(!L[cur.pos].empty()){
				int w = L[cur.pos].back().first;
				if(w > cur.last){
					Q.push(node(L[cur.pos].back().second,1,w,cur.distance + w));
					L[cur.pos].pop_back();
				}else break;
			}
		}
	}
	return -1;
}

int main(){
	int T,N,M,source,sink;
	scanf("%d",&T);
	while(T--){
		scanf("%d %d",&N,&M);
		for(int i = 0;i < N;++i){
			L[i].clear();
			L2[i].clear();
		}
		for(int i = 0,u,v,w;i < M;++i){
			scanf("%d %d %d",&u,&v,&w);
			--u; --v;
			L[u].push_back(make_pair(w,v));
			L[v].push_back(make_pair(w,u));
			L2[u].push_back(make_pair(w,v));
			L2[v].push_back(make_pair(w,u));
		}
		for(int i = 0;i < N;++i){
			sort(L[i].begin(),L[i].end());
			sort(L2[i].rbegin(),L2[i].rend());  //贪心思想，奇数位从大到小，偶数位从小到大，这样也不会相撞
		}
		scanf("%d %d",&source,&sink);
		--source; --sink;
		long long ret = dijkstra(source,sink);
		if(ret == -1) printf("No Solution\n");
		else printf("%lld\n",ret);
	}
	return 0;
}