大意:一群牛排成一队,其中某些牛需要在一定的距离内,而有些则要离开一定的距离。
思路:差分约束。
s[i]-s[i-1] <= d
s[i]-s[i-1]>=d
s[i]-s[i-1]>=0
如果有环,输出-1,如果N可以无限远,即1与N不连通,输出-2,其他情况输出1与N的最大距离。
CODE:
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
const int SIZE = 100010;
const int INF = 0x3f3f3f3f;
int u[5*SIZE], w[5*SIZE], v[5*SIZE], next[5*SIZE];
int first[SIZE], d[SIZE];
int n, ML, MD,cnt;
int sum[SIZE];
void read_graph(int u1, int v1, int w1)
{
u[cnt] = u1, v[cnt] = v1, w[cnt] = w1;
next[cnt] = first[u[cnt]];
first[u[cnt]] = cnt++;
}
int spfa(int src)
{
queue<int> q;
bool inq[SIZE] = {0};
for(int i = 0; i <= n; i++) d[i] = (i == src)? 0:INF;
q.push(src);
while(!q.empty())
{
int x = q.front(); q.pop();
inq[x] = 0;
for(int e = first[x]; e!=-1; e = next[e]) if(d[v[e]] > d[x]+w[e])
{
d[v[e]] = d[x]+w[e];
if(!inq[v[e]])
{
inq[v[e]] = 1;
sum[v[e]]++;
if(sum[v[e]] >= n)
{
return -1;
}
q.push(v[e]);
}
}
}
if(d[n] == INF) return -2;
else return d[n];
}
void init()
{
memset(first, -1, sizeof(first));
memset(sum, 0, sizeof(sum));
cnt = 0;
}
int main()
{
int u1, v1, w1;
while(~scanf("%d%d%d", &n, &ML, &MD))
{
init();
for(int i = 1; i <= ML; i++)
{
scanf("%d%d%d", &u1, &v1, &w1);
read_graph(u1, v1, w1);
}
for(int i = 1; i <= MD; i++)
{
scanf("%d%d%d", &u1, &v1, &w1);
read_graph(v1, u1, -w1);
}
for(int i = 1; i <= n; i++)
read_graph(i, i-1, 0);
int ans = spfa(1);
printf("%d\n", ans);
}
return 0;
}
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
const int SIZE = 100010;
const int INF = 0x3f3f3f3f;
int u[5*SIZE], w[5*SIZE], v[5*SIZE], next[5*SIZE];
int first[SIZE], d[SIZE];
int n, ML, MD,cnt;
int sum[SIZE];
void read_graph(int u1, int v1, int w1)
{
u[cnt] = u1, v[cnt] = v1, w[cnt] = w1;
next[cnt] = first[u[cnt]];
first[u[cnt]] = cnt++;
}
int spfa(int src)
{
queue<int> q;
bool inq[SIZE] = {0};
for(int i = 0; i <= n; i++) d[i] = (i == src)? 0:INF;
q.push(src);
while(!q.empty())
{
int x = q.front(); q.pop();
inq[x] = 0;
for(int e = first[x]; e!=-1; e = next[e]) if(d[v[e]] > d[x]+w[e])
{
d[v[e]] = d[x]+w[e];
if(!inq[v[e]])
{
inq[v[e]] = 1;
sum[v[e]]++;
if(sum[v[e]] >= n)
{
return -1;
}
q.push(v[e]);
}
}
}
if(d[n] == INF) return -2;
else return d[n];
}
void init()
{
memset(first, -1, sizeof(first));
memset(sum, 0, sizeof(sum));
cnt = 0;
}
int main()
{
int u1, v1, w1;
while(~scanf("%d%d%d", &n, &ML, &MD))
{
init();
for(int i = 1; i <= ML; i++)
{
scanf("%d%d%d", &u1, &v1, &w1);
read_graph(u1, v1, w1);
}
for(int i = 1; i <= MD; i++)
{
scanf("%d%d%d", &u1, &v1, &w1);
read_graph(v1, u1, -w1);
}
for(int i = 1; i <= n; i++)
read_graph(i, i-1, 0);
int ans = spfa(1);
printf("%d\n", ans);
}
return 0;
}