POJ 3204 Ikki's Story I - Road Reconstruction

  1 #include <cstdio>
  2 #include <cstring>
  3 
  4 #define fread_siz 1024
  5 
  6 inline int get_c(void)
  7 {
  8     static char buf[fread_siz];
  9     static char *head = buf + fread_siz;
 10     static char *tail = buf + fread_siz;
 11 
 12     if (head == tail)
 13         fread(head = buf, 1, fread_siz, stdin);
 14 
 15     return *head++;
 16 }
 17 
 18 inline int get_i(void)
 19 {
 20     register int ret = 0;
 21     register int neg = false;
 22     register int bit = get_c();
 23 
 24     for (; bit < 48; bit = get_c())
 25         if (bit == ‘-‘)neg ^= true;
 26 
 27     for (; bit > 47; bit = get_c())
 28         ret = ret * 10 + bit - 48;
 29 
 30     return neg ? -ret : ret;
 31 }
 32 
 33 inline int min(int a, int b)
 34 {
 35     return a < b ? a : b;
 36 }
 37 
 38 const int inf = 2e9;
 39 const int maxn = 200005;
 40 
 41 int n, m;
 42 int s, t;
 43 int edges;
 44 int hd[maxn];
 45 int to[maxn];
 46 int fl[maxn];
 47 int nt[maxn];
 48 
 49 inline void add(int u, int v, int f)
 50 {
 51     nt[edges] = hd[u]; to[edges] = v; fl[edges] = f; hd[u] = edges++;
 52     nt[edges] = hd[v]; to[edges] = u; fl[edges] = 0; hd[v] = edges++;
 53 }
 54 
 55 int dep[maxn];
 56 
 57 inline bool bfs(void)
 58 {
 59     static int que[maxn];
 60     static int head, tail;
 61     
 62     memset(dep, 0, sizeof(dep));
 63     head = 0, tail = 0;
 64     que[tail++] = s;
 65     dep[s] = 1;
 66     
 67     while (head != tail)
 68     {
 69         int u = que[head++], v;
 70         for (int i = hd[u]; ~i; i = nt[i])
 71             if (!dep[v = to[i]] && fl[i])
 72                 dep[v] = dep[u] + 1, que[tail++] = v;
 73     }
 74     
 75     return dep[t];
 76 }
 77 
 78 int dfs(int u, int f)
 79 {
 80     if (u == t || !f)
 81         return f;
 82         
 83     int used = 0, flow;
 84     
 85     for (int i = hd[u], v; ~i; i = nt[i])
 86         if (dep[v = to[i]] == dep[u] + 1 && fl[i])
 87         {
 88             flow = dfs(v, min(f - used, fl[i]));
 89             
 90             used += flow;
 91             fl[i] -= flow;
 92             fl[i^1] += flow;
 93             
 94             if (used == f)
 95                 return f;
 96         }
 97         
 98     if (!used)
 99         dep[u] = 0;
100         
101     return used;
102 }
103 
104 inline bool dfs(void)
105 {
106     return dfs(s, inf) != 0;
107 }
108 
109 inline void maxFlow(void)
110 {
111     while (bfs())
112         while (dfs());
113 }
114 
115 int dfn[maxn];
116 int scc[maxn];
117 
118 void tarjan(int u)
119 {
120     static int low[maxn];
121     static int stk[maxn];
122     static int top, cnt, tim;
123     
124     stk[++top] = u;
125     dfn[u] = low[u] = ++tim;
126     
127     for (int i = hd[u]; ~i; i = nt[i])if (fl[i])
128     {
129         if (!dfn[to[i]])
130             tarjan(to[i]), low[u] = min(low[u], low[to[i]]);
131         else if (!scc[to[i]])
132             low[u] = min(low[u], dfn[to[i]]);
133     }
134     
135     if (low[u] == dfn[u])
136     {
137         ++cnt; int t;
138         do
139             scc[t = stk[top--]] = cnt;
140         while (t != u);
141     }
142 }
143 
144 inline void tarjan(void)
145 {
146     for (int i = 0; i < n; ++i)
147         if (!dfn[i])tarjan(i);
148 }
149 
150 inline void solve(void)
151 {
152     int ans = 0;
153     
154     for (int i = 0; i < edges; i += 2)if (!fl[i])
155     {
156         int u = to[i^1], v = to[i];
157         if (scc[u] == scc[s])
158             if (scc[v] == scc[t])
159                 ++ans;
160     }
161     
162     printf("%d\n", ans);
163 }
164 
165 signed main(void)
166 {
167     n = get_i();
168     m = get_i();
169     
170     memset(hd, -1, sizeof(hd));
171     
172     for (int i = 1; i <= m; ++i)
173     {
174         int u = get_i();
175         int v = get_i();
176         int f = get_i();
177         
178         add(u, v, f);
179     }
180     
181     s = 0, t = n - 1;
182     
183     maxFlow();
184     
185     tarjan();
186     
187     solve();
188 }