Silver Cow Party
Time Limit: 2000MS | Memory Limit: 65536K | |
Total Submissions: 13183 | Accepted: 5932 |
Description
One cow from each of N farms (1 ≤ N ≤ 1000) conveniently numbered 1..N is going to attend the big cow party to be held at farm #X (1 ≤ X ≤ N). A total of M (1 ≤ M ≤ 100,000) unidirectional
(one-way roads connects pairs of farms; road i requires Ti (1 ≤ Ti ≤ 100) units of time to traverse.
Each cow must walk to the party and, when the party is over, return to her farm. Each cow is lazy and thus picks an optimal route with the shortest time. A cow‘s return route might be different from her original route to the party since roads are one-way.
Of all the cows, what is the longest amount of time a cow must spend walking to the party and back?
Input
Line 1: Three space-separated integers, respectively: N, M, and X
Lines 2..M+1: Line i+1 describes road i with three space-separated integers: Ai, Bi, and Ti. The described road runs from farm Ai to farm Bi,
requiring Ti time units to traverse.
Output
Line 1: One integer: the maximum of time any one cow must walk.
Sample Input
4 8 2 1 2 4 1 3 2 1 4 7 2 1 1 2 3 5 3 1 2 3 4 4 4 2 3
Sample Output
10
Hint
Cow 4 proceeds directly to the party (3 units) and returns via farms 1 and 3 (7 units), for a total of 10 time units.
Source
因为是求一个来回,所以构造顺逆两张图,两次Dijkstra即可。
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <iostream> #include <algorithm> #include <queue> using namespace std; typedef long long LL; #define maxn 1010 #define maxm 100010 #define inf 0x3f3f3f3f int N, M, X; int head1[maxn], head2[maxn], id1, id2; struct Node { int v, w, next; } E1[maxm], E2[maxm]; int dist1[maxn], dist2[maxn]; bool vis1[maxn], vis2[maxn]; void addEdge(int head[], Node E[], int& id, int u, int v, int w) { E[id].v = v; E[id].w = w; E[id].next = head[u]; head[u] = id++; } void getMap() { int u, v, w; id1 = id2 = 1; while(M--) { scanf("%d%d%d", &u, &v, &w); addEdge(head1, E1, id1, u, v, w); addEdge(head2, E2, id2, v, u, w); // reverse } } int getNext(int dist[], bool vis[]) { int u = 0, i, v = inf; for(i = 1; i <= N; ++i) if(!vis[i] && dist[i] < v) { v = dist[i]; u = i; } return u; } void Dijkstra(int source, int head[], Node E[], int dist[], bool vis[]) { int i, u = source, v, w; memset(dist, 0x3f, sizeof(int) * (N + 1)); dist[u] = 0; while(u) { vis[u] = 1; for(i = head[u]; i; i = E[i].next) dist[v=E[i].v] = min(dist[v], dist[u] + E[i].w); u = getNext(dist, vis); } } int main() { // freopen("stdin.txt", "r", stdin); scanf("%d%d%d", &N, &M, &X); getMap(); Dijkstra(X, head1, E1, dist1, vis1); Dijkstra(X, head2, E2, dist2, vis2); int ans = 0; for(int i = 1; i <= N; ++i) ans = max(ans, dist1[i] + dist2[i]); printf("%d\n", ans); return 0; }