Jzzhu and Cities CodeForces - 450D

题目链接 

思路： 其实本题就是给你一些假设的最短路，问这些最短路是否可以被进一步变短。

一开始没有去考虑到，已经成为最短路的点可以去松弛其他的点。

#include <cstring>
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <queue>

using namespace std;

const int N = 1e5+10;
const long long inf = 1e18;
struct node {
long long dis;
int index;
node(int x,long long y ) {
index = x; dis = y ;
}
bool operator < (const node &a) const {
return dis>a.dis;
}
};
struct Edge{
int to;
long long dis;
};
Edge edge[N*6];
bool vis
;
long long dis
;
priority_queue<node> que;
int n,m,k;
int head
;
int nex[6*N];
int cnt=0;
bool flag
;

void add(int x,int y,long long w ) {
edge[cnt].to = y;
edge[cnt].dis = w;
nex[cnt] = head[x];
head[x] = cnt;

++cnt;
edge[cnt].to = x;
edge[cnt].dis = w;
nex[cnt] = head[y];
head[y] = cnt;

++cnt;
}
int main()
{
int a,b;
int count = 0;
int tx;
long long w;
node temp(0,0);

cin>>n>>m>>k;
memset( head,-1,sizeof(head) );
memset( vis,false,sizeof(vis) );

for ( int i=1; i<=n;i++ )
dis[i] = inf;
for ( int i=0; i<m; i++ ) {
scanf("%d%d",&a,&b); cin>>w;
add( a,b,w );
}

for ( int i=0; i<k; i++ ) {
scanf("%d",&a);cin>>w;
flag[a] = true ;
if ( dis[a]==inf ) dis[a] = w;
else {
count++;
if ( w<dis[a] ) dis[a] = w;
}
}
for ( int i=1; i<=n; i++ ) {
if ( dis[i]!=inf ) {
que.push(node(i,dis[i]));
}
}

dis[1] = 0;
que.push(node(1,0));
while( !que.empty() ) {
temp = que.top();
que.pop();
tx = temp.index;

if ( vis[tx]==false ) {
vis[tx] = 1 ;
for ( int i=head[tx]; i!=-1 ; i=nex[i] ) {
if ( dis[tx]+edge[i].dis <= dis[edge[i].to] ) {
if ( flag[edge[i].to] ) {
flag[edge[i].to]=0;
++count;
}
dis[edge[i].to] = dis[tx]+edge[i].dis;
que.push( node( edge[i].to , dis[edge[i].to]));
}
}
}
}
cout<<count<<endl;
return 0;
}