題目鏈接:UVALive 6378
問題描述:
一開始讀題沒讀懂,後來也理解錯了……
多源最短路,floyd硬槓會超時(就算強行優化也會超時Orz)。spfa單源最短路算法循環一遍所有起點,鄰接鏈表存邊避免遍歷大量無用點(直接用鄰接矩陣存圖,樸素spfa仍然會超時),鑑於題目中的用字符串描述對應關係,用map的特性優化。
有不少人是200+ms過的,可能是vector用時更短,vector不能存放邊的權值,在這道題中使用沒有什麼問題。嘗試改動代碼後運行時間從850ms縮減到300+ms,其他的可能有初始化使用的memset比較費時等等,沒有再嘗試。
一道題讓我學會了用鄰接鏈表減少遍歷次數+map容器(一個耿直的笑【我好渣啊】)。
手寫鄰接鏈表:
#include <iostream>
#include <string>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <ctime>
#include <queue>
#include <map>
#define MAX 20005
#define M 1005
#define INF 0x3f3f3f3f
using namespace std;
map<string, int> name;
int dist[M], head[M], cnt;
bool vis[M];
struct node
{
int u, v, w, nxt;
}edge[MAX];
void add(int u, int v, int w)
{
edge[cnt].u = u;
edge[cnt].v = v;
edge[cnt].w = w;
edge[cnt].nxt = head[u];
head[u] = cnt++;
edge[cnt].u = v;
edge[cnt].v = u;
edge[cnt].w = w;
edge[cnt].nxt = head[v];
head[v] = cnt++;
}
void spfa(int s)
{
memset(dist, INF, sizeof(dist));
memset(vis, 0, sizeof(vis));
dist[s] = 0;
vis[s] = true;
queue<int> Q;
Q.push(s);
while(!Q.empty())
{
int u = Q.front();
Q.pop();
vis[u] = false;
for(int i = head[u]; i != -1; i = edge[i].nxt)
{
int v = edge[i].v;
if(dist[u] + edge[i].w < dist[v])
{
dist[v] = dist[u] + edge[i].w;
if(!vis[v])
{
vis[v] = true;
Q.push(v);
}
}
}
}
}
int main()
{
int n, m;
string s, t1, t2;
while(scanf("%d", &n) && n)
{
cnt = 0;
name.clear();
memset(head, -1, sizeof(head));
for(int i = 0; i < n; i++)
{
cin >> s;
name[s] = i;//map查找優化
}
scanf("%d", &m);
while(m--)
{
cin >> t1 >> t2;
add(name[t1], name[t2], 1);
}
int Max = 0;
for(int i = 0; i < n; i++)
{
spfa(i);
for(int j = 0; j < n; j++)
{
Max = max(Max, dist[j]);
if(Max == INF)
break;
}
if(Max == INF)
break;
}
if(Max != INF)
printf("%d\n", Max);
else
printf("-1\n");
}
return 0;
}
hust Vjudge運行結果:
vector實現:
#include <iostream>
#include <string>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <ctime>
#include <queue>
#include <map>
#define M 2005
#define INF 0x3f3f3f3f
using namespace std;
map<string, int> name;
int dist[M], cnt;
bool vis[M];
vector<int> edge[M];
void spfa(int s)
{
memset(dist, INF, sizeof(dist));
memset(vis, 0, sizeof(vis));
dist[s] = 0;
vis[s] = true;
queue<int> Q;
Q.push(s);
while(!Q.empty())
{
int u = Q.front();
Q.pop();
vis[u] = false;
for(int i = 0; i < edge[u].size(); i++)
{
int v = edge[u][i];
if(dist[u] + 1 < dist[v])
{
dist[v] = dist[u] + 1;
if(!vis[v])
{
vis[v] = true;
Q.push(v);
}
}
}
}
}
int main()
{
int n, m;
string s, t1, t2;
while(scanf("%d", &n) && n)
{
cnt = 0;
name.clear();
for(int i = 0; i < n; i++)//vector初始化
edge[i].clear();
for(int i = 0; i < n; i++)
{
cin >> s;
name[s] = i;//map查找優化
}
scanf("%d", &m);
while(m--)
{
cin >> t1 >> t2;
edge[name[t1]].push_back(name[t2]);
edge[name[t2]].push_back(name[t1]);
}
int Max = 0;
for(int i = 0; i < n; i++)
{
spfa(i);
for(int j = 0; j < n; j++)
{
Max = max(Max, dist[j]);
if(Max == INF)
break;
}
if(Max == INF)
break;
}
if(Max != INF)
printf("%d\n", Max);
else
printf("-1\n");
}
return 0;
}
hust Vjudge運行結果: