BZOJ 2521: [Shoi2010]最小生成树

  1 #include <cstdio>
  2 #include <cstring>
  3 
  4 inline char nextChar(void)
  5 {
  6     static const int siz = 1 << 20;
  7     
  8     static char buf[siz];
  9     static char *hd = buf + siz;
 10     static char *tl = buf + siz;
 11     
 12     if (hd == tl)
 13         fread(hd = buf, 1, siz, stdin);
 14     
 15     return *hd++;
 16 }
 17 
 18 inline int nextInt(void)
 19 {
 20     register int ret = 0;
 21     register bool neg = false;
 22     register char bit = nextChar();
 23     
 24     for (; bit < 48; bit = nextChar());
 25         if (bit == ‘-‘)neg ^= true;
 26     
 27     for (; bit > 47; bit = nextChar())
 28         ret = ret * 10 + bit - ‘0‘;
 29     
 30     return neg ? -ret : ret;
 31 }
 32 
 33 const int siz = 2005;
 34 const int edg = 2e6 + 5;
 35 const int inf = 2e9 + 7;
 36 
 37 int n, m, id, s, t;
 38 
 39 struct edge
 40 {
 41     int x, y, w;
 42 }e[edg];
 43 
 44 int hd[siz], to[edg], nt[edg], fl[edg], tot;
 45 
 46 inline void add(int u, int v, int f)
 47 {
 48     nt[tot] = hd[u]; to[tot] = v; fl[tot] = f; hd[u] = tot++;
 49     nt[tot] = hd[v]; to[tot] = u; fl[tot] = 0; hd[v] = tot++;
 50 }
 51 
 52 int dep[siz];
 53 
 54 inline bool bfs(void)
 55 {
 56     static int que[siz];
 57     static int head, tail;
 58     
 59     memset(dep, 0, sizeof(dep));
 60     
 61     que[head = 0] = s, tail = dep[s] = 1;
 62     
 63     while (head != tail)
 64     {
 65         int u = que[head++], v;
 66         
 67         for (int i = hd[u]; ~i; i = nt[i])
 68             if (!dep[v = to[i]] && fl[i])
 69                 dep[que[tail++] = v] = dep[u] + 1;
 70     }
 71     
 72     return dep[t];
 73 }
 74 
 75 int cur[siz];
 76 
 77 inline int min(int a, int b)
 78 {
 79     return a < b ? a : b;
 80 }
 81 
 82 int dfs(int u, int f)
 83 {
 84     if (!f || u == t)
 85         return f;
 86     
 87     int used = 0, flow, v;
 88     
 89     for (int i = cur[u]; ~i; i = nt[i])
 90         if (dep[v = to[i]] == dep[u] + 1 && fl[i])
 91         {
 92             flow = dfs(v, min(fl[i], f - used));
 93             
 94             used += flow;
 95             fl[i] -= flow;
 96             fl[i^1] += flow;
 97             
 98             if (fl[i])
 99                 cur[u] = i;
100             
101             if (used == f)
102                 return f;
103         }
104     
105     if (!used)
106         dep[u] = 0;
107     
108     return used;
109 }
110 
111 inline int minCut(void)
112 {
113     int minCut = 0, newFlow;
114     
115     while (bfs())
116     {
117         memcpy(cur, hd, sizeof(hd));
118         
119         while (newFlow = dfs(s, inf))
120             minCut += newFlow;
121     }
122     
123     return minCut;
124 }
125 
126 signed main(void)
127 {
128     n = nextInt();
129     m = nextInt();
130     
131     id = nextInt();
132     
133     for (int i = 1; i <= m; ++i)
134     {
135         e[i].x = nextInt();
136         e[i].y = nextInt();
137         e[i].w = nextInt();
138     }
139     
140     s = e[id].x;
141     t = e[id].y;
142     
143     int lim = e[id].w;
144     
145     memset(hd, -1, sizeof(hd));
146     
147     for (int i = 1; i <= m; ++i)
148         if (e[i].w <= lim && i != id)
149             add(e[i].x, e[i].y, lim + 1 - e[i].w),
150             add(e[i].y, e[i].x, lim + 1 - e[i].w);
151         
152     printf("%d\n", minCut());
153 }