Telephone Lines USACO 月赛

以前做过这套题目

这个题又重新写了：/article/5825629.html

还是以前的思路

一直错在一个地方：决策的时候，如果没有走过，直接更新，如果走过，总是选最小值。如果走的是小于mid值的边，那么用tmp = max(dp[x][u] , e[i].val) 更新。

代码比以前好多了好像、

#define maxn 1005

int dp[maxn][maxn];
int n,m,k;
struct node
{
int v,next;
int val;
int f;
};
node e[10005 * 2 ];
int head[maxn];
int cnt ;
void init()
{
cnt = 0 ;
memset(head,-1,sizeof(head));
}
void add(int u,int v,int w)
{
e[cnt].v = v ;
e[cnt].val = w;
e[cnt].next = head[u] ;
head[u] = cnt ++ ;

e[cnt].v = u ;
e[cnt].val = w ;
e[cnt].next = head[v];
head[v] = cnt ++ ;
return ;
}
bool vis[maxn];

void dfs(int u)
{
vis[u]= 1 ;
for(int i =  head[u]; i != -1 ; i = e[i].next )
if(!vis[e[i].v] )
dfs(e[i].v);
}

queue< pair<int,int> > q;
#define pii pair<int,int>
bool vs[maxn][maxn];

void spfa()
{
pii cur,tt;
int u , x;
while(!q.empty()) q.pop();
dp[1][0] = 0 ;
vs[1][0] = 1;
q.push(make_pair(1,0));
while(!q.empty())
{
cur = q.front();
q.pop();
u = cur.first;
x = cur.second;
for(int i = head[u]; i != -1 ; i = e[i].next )
{
if(e[i].f == 1 )
{
tt = make_pair(e[i].v , x + 1 );
if(x + 1 > k ) continue;
// printf("%d %d %d %d\n",u,x,e[i].v,x+1);
//int tmp = max(dp[e[i].v][x+1],dp[u][x]);
if((dp[e[i].v][x+1] == -1 && dp[u][x] >= 0  )|| ( dp[e[i].v][x+1] != -1 && dp[u][x] >= 0 && dp[u][x] < dp[e[i].v][x+1] ) )
{
dp[e[i].v][x+1] = dp[u][x] ;
if(!vs[tt.first][tt.second])
{
vs[tt.first][tt.second] = 1 ;
q.push(tt);
}
}
}
else if(e[i].f == 0 )
{
tt = make_pair(e[i].v,x);
if(x > k ) continue;
// printf("%d %d %d %d\n",u,x,e[i].v,x);
int tmp = max(e[i].val , dp[u][x] );
if(dp[e[i].v][x] == -1 || tmp < dp[e[i].v][x])
{
dp[e[i].v][x] = tmp;
if(!vs[tt.first][tt.second])
{
vs[tt.first][tt.second] = 1 ;
q.push(tt);
}
}
}
}
vs[u][x] = 0;
}
//if(dp
[k] >= 0 ) puts("asjklsajk");
}
int check()
{
memset(dp,-1,sizeof(dp));
memset(vs,0,sizeof(vs));
spfa();
//printf("dp
[k] = %d\n",dp
[k]);
return dp
[k];
}
void build(int x)
{
for(int i = 1; i <= n ; i ++ )
{
for(int j = head[i] ; j != -1 ; j = e[j].next)
if(e[j].val > x )
e[j].f = 1;
else e[j].f = 0 ;
}
}
int _max;
int main()
{
int u,v,w;
while(scanf("%d%d%d",&n,&m,&k)!=EOF)
{
init();
_max = 0 ;
for(int i = 1 ; i <= m ; i ++ )
{
scanf("%d%d%d",&u,&v,&w);
_max = max(_max , w);
add(u,v,w);
}
dfs(1);
if(!vis
)
{
printf("-1\n");
continue;
}
int left , right ,mid ;
left = 1 ;
right = _max ;
int ans ;
int res;
ans = 0x3f3f3f3f;
while(left < right)
{
mid = (left + right ) / 2  ;
//printf("mid = %d\n",mid);
build(mid);
res = check();
//printf("res = %d\n",res);
if(res == -1 )
{
// printf("%d\n",left);
left = mid + 1 ;
}
else
{
ans = min(ans , res);
right = mid ;
}
}
printf("%d\n",ans);
}
return 0;
}

/*
8 10 2
1 2 1
1 3 10
2 3 2
3 4 11
3 5 3
4 5 10
4 8 12
4 6 9
5 7 4
6 7 5
*/

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