今日头条”杯2018年湖北省赛网）

Posted 2020-10-31 chaoswr

所有题目： http://cdn.vo-ov.cn/online_f9ec217.pdf

 

F: A-maze-ing

哇我也是哭了...dfs写错，dfs还用了vis数组，实际上并不需要，WA了N多次...呜呜呜

看出来对图的基本概念还比较生疏，或者说都忘了好多，一开始还在纠结环是不是强连通分量...唉如果能果断点就好了

  1 #include <iostream>
  2 #include <string.h>
  3 #include <cstdio>
  4 #include <queue>
  5 #include <set>
  6 #include <cstring>
  7 #include <algorithm>
  8 
  9 #define SIGMA_SIZE 26
 10 #pragma warning ( disable : 4996 )
 11 
 12 using namespace std;
 13 typedef long long LL;
 14 
 15 inline LL LMax(LL a,LL b)    { return a>b?a:b; }
 16 inline LL LMin(LL a,LL b)    { return a>b?b:a; }
 17 inline int Max(int a,int b) { return a>b?a:b; }
 18 inline int Min(int a,int b) { return a>b?b:a; }
 19 inline int gcd( int a, int b ) { return b==0?a:gcd(b,a%b); }
 20 inline int lcm( int a, int b ) { return a/gcd(a,b)*b; }  //a*b = gcd*lcm
 21 const int inf  = 0x3f3f3f3f;
 22 const int mod  = 7;
 23 const int maxn = 2e5+5;
 24 const int maxk = 200;
 25 
 26 struct edge {
 27     int to, next;
 28 }edges[maxn];
 29 
 30 struct edg{
 31     int to, next;
 32 }e[maxn];
 33 
 34 int N, M, cnt, sum;
 35 int order, iindex;
 36 int linjie[maxn], linjie_2[maxn], indeg[maxn], LOW[maxn];
 37 int DFN[maxn], belong[maxn], Stack[maxn];
 38 int num[maxn];
 39 bool vis[maxn];
 40 
 41 
 42 void addedge( int u, int v )
 43 {
 44     edges[cnt].to = v; edges[cnt].next = linjie[u]; linjie[u] = cnt++; }
 45 
 46 void addedge_2( int u, int v )
 47 { 
 48     e[cnt].to = v; e[cnt].next = linjie_2[u]; linjie_2[u] = cnt++; }
 49 
 50 void init()
 51 {
 52     cnt = order = iindex = sum = 0;
 53     memset( linjie, -1, sizeof(linjie) );
 54     memset( linjie_2, -1, sizeof(linjie_2) );
 55     memset( indeg, 0, sizeof(indeg) );
 56     memset( LOW, 0, sizeof(LOW) );
 57     memset( DFN, 0, sizeof(DFN) );
 58     memset( belong, 0, sizeof(belong) );
 59     memset( Stack, 0, sizeof(Stack) );
 60     memset( num, 0, sizeof(num) );
 61     memset( vis, 0, sizeof(vis) );
 62 }
 63 
 64 void tarjan( int x )
 65 {
 66     DFN[x] = LOW[x] = ++order;
 67     Stack[++iindex] = x;
 68     vis[x] = true;
 69     for( int i = linjie[x]; i+1; i = edges[i].next )
 70     {
 71         if (!DFN[edges[i].to])
 72         {
 73             tarjan(edges[i].to);
 74             LOW[x] = Min( LOW[x], LOW[edges[i].to] );
 75         }
 76         else if ( vis[edges[i].to] )
 77             LOW[x] = Min( LOW[x], DFN[edges[i].to] );
 78     }
 79 
 80     if ( LOW[x] == DFN[x] )
 81     {
 82         sum++;
 83         while( iindex )
 84         {
 85 
 86             int temp = Stack[iindex--];
 87             vis[temp] = false;
 88             belong[temp] = sum;
 89             if ( temp == x )
 90                 break;
 91         }
 92     }
 93 }
 94 
 95 void solve()
 96 {
 97     cnt = 0;
 98 
 99     for ( int i = 1; i <= N; i++ )
100         num[belong[i]]++;
101 
102     for ( int i = 1; i <= N; i++ )
103         for ( int j = linjie[i]; j+1; j = edges[j].next )
104             if ( belong[i] != belong[edges[j].to] )
105             {
106                 indeg[belong[edges[j].to]] = 1;
107                 addedge_2(belong[i], belong[edges[j].to]);
108             }
109 }
110 
111 void read()
112 {
113     int x, y;
114     for ( int i = 1; i <= M; i++ )
115     { 
116         scanf("%d %d", &x, &y); 
117         addedge(x, y);
118     }
119 }
120 
121 void find( int x )
122 {
123     if ( linjie_2[x] == -1 )
124     {
125         belong[x] = num[x];
126         return;
127     }
128 
129 
130     for ( int i = linjie_2[x]; i+1; i = e[i].next )
131     {
132         if (!belong[e[i].to])
133             find(e[i].to);
134         belong[x] = Max( belong[x], belong[e[i].to]+num[x] );
135     }
136     
137     return;
138 }
139 
140 int main()
141 {
142     while (~scanf("%d %d", &N, &M))
143     {
144         init();
145         read();
146 
147         for (int i = 1; i <= N; i++)
148             if (!DFN[i])
149                 tarjan(i);
150         solve();
151 
152         int ans = 0;
153         memset( belong, 0, sizeof(belong) );
154 
155         for (int i = 1; i <= sum; i++)
156             if (!indeg[i])
157             {
158                 find(i);
159                 ans = Max(ans, belong[i]);
160             }
161 
162         printf("%d\n", ans);
163     }
164     return 0;
165 }

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