最大流

 1 //isap
 2 struct isap_edge{
 3     int from, to, next, val;
 4 }isap_e[MAXM];
 5 
 6 int isap_cnt, isap_h[MAXN];
 7 
 8 void isap_init(){
 9     isap_cnt = 0;
10     memset( isap_h, -1, sizeof( isap_h ) );
11 }
12 
13 void isap_add( int from, int to, int v1, int v2 ){
14     isap_e[isap_cnt].from = from;
15     isap_e[isap_cnt].to = to;
16     isap_e[isap_cnt].val = v1;
17     isap_e[isap_cnt].next = isap_h[from];
18     isap_h[from] = isap_cnt++;
19 
20     swap( from, to );
21 
22     isap_e[isap_cnt].from = from;
23     isap_e[isap_cnt].to = to;
24     isap_e[isap_cnt].val = v2;
25     isap_e[isap_cnt].next = isap_h[from];
26     isap_h[from] = isap_cnt++;
27 }
28 
29 int isap( int s, int t, int n ){
30     int Q[MAXN], cnt[MAXN], d[MAXN], low[MAXN], cur[MAXN];
31 
32     int l = 0, r = 0;
33     for( int i = 1; i <= n; i++ ){
34         d[i] = n; cnt[i] = 0;
35     }
36     cnt[n] = n - 1; cnt[0]++; d[t] = 0;
37 
38     Q[r++] = t;
39     while( l < r ){
40         int v = Q[l++];
41         for( int i = isap_h[v]; i != -1; i = isap_e[i].next ){
42             if( d[isap_e[i].to] == n && isap_e[i ^ 1].val > 0 ){
43                 d[isap_e[i].to] = d[v] + 1;
44                 cnt[n]--;
45                 cnt[d[isap_e[i].to]]++;
46                 Q[r++] = isap_e[i].to;
47             }
48         }
49     }
50 
51     int max_flow = 0, pos = s, top = 0; low[0] = INF;
52     for( int i = 1; i <= n; i++ )
53         cur[i] = isap_h[i];
54     while( d[s] < n ){
55         int &i = cur[pos];
56         for( ; i != -1; i = isap_e[i].next ){
57             if( isap_e[i].val > 0 && d[pos] == d[isap_e[i].to] + 1 ){
58                 low[top + 1] = min( low[top], isap_e[i].val );
59                 Q[++top] = i;
60                 pos = isap_e[i].to;
61                 break;
62             }
63         }
64 
65         if( i != -1 ){
66             if( pos == t ){
67                 int minf = low[top];
68                 for( int p = 1,i; p <= top; ++p ){
69                     i = Q[p];
70                     isap_e[i].val -= minf;
71                     isap_e[i ^ 1].val += minf;
72                 }
73                 max_flow += minf;
74                 pos = s;
75                 low[0] = INF;
76                 top = 0;
77             }
78         }
79         else{
80             int old = d[pos];
81             cnt[old]--;
82             d[pos] = n - 1;
83             for( int i = isap_h[pos]; i != -1; i = isap_e[i].next )
84                 if( isap_e[i].val > 0 && d[pos] > d[isap_e[i].to] )
85                     d[pos] = d[isap_e[i].to];
86             cnt[++d[pos]]++;
87             if( d[pos] < n )
88                     cur[pos] = isap_h[pos];
89             if( pos != s ){
90                 pos = isap_e[Q[top]].from;
91                 --top;
92             }
93             if( cnt[old] == 0 )
94                 break;
95         }
96     }
97     return max_flow;
98 }