isap+bfs初始化+栈优化，点的编号从0开始：
1 const int MAXN = 100010; 2 const int MAXM = 400010; 3 const int INF = 0x3f3f3f3f; 4 struct Edge 5 { 6 int to, next, cap, flow; 7 }edge[MAXM]; 8 int tol; 9 int head[MAXN]; 10 int gap[MAXN], dep[MAXN], cur[MAXN]; 11 void init() 12 { 13 tol = 0; 14 memset(head, -1, sizeof(head)); 15 } 16 void addedge(int u, int v, int w, int rw = 0) 17 { 18 edge[tol].to = v; edge[tol].cap = w; edge[tol].flow = 0; 19 edge[tol].next = head[u]; head[u] = tol++; 20 edge[tol].to = u; edge[tol].cap = rw; edge[tol].flow = 0; 21 edge[tol].next = head[v]; head[v] = tol++; 22 } 23 int Q[MAXN]; 24 void BFS(int start, int end) 25 { 26 memset(dep, -1, sizeof(dep)); 27 memset(gap, 0, sizeof(gap)); 28 gap[0] = 1; 29 int front = 0, rear = 0; 30 dep[end] = 0; 31 Q[rear++] = end; 32 while(front != end) { 33 int u = Q[front++]; 34 for(int i = head[u]; i != -1; i = edge[i].next) { 35 int v = edge[i].to; 36 if(dep[v] != -1) continue; 37 Q[rear++] = v; 38 dep[v] = dep[u] + 1; 39 gap[dep[v]]++; 40 } 41 } 42 } 43 int S[MAXN]; 44 int sap(int start, int end, int N) { //start是源点编号，end是汇点编号，N是点的总数，返回最大流 45 BFS(start, end); 46 memcpy(cur, head, sizeof(head)); 47 int top = 0; 48 int u = start; 49 int ans = 0; 50 while(dep[start] < N) 51 { 52 if(u == end) 53 { 54 int Min = INF; 55 int inser; 56 for(int i = 0; i < top; i++) 57 if(Min > edge[S[i]].cap - edge[S[i]].flow) { 58 Min = edge[S[i]].cap - edge[S[i]].flow; 59 inser = i; 60 } 61 for(int i = 0; i < top; i++) 62 { 63 edge[S[i]].flow += Min; 64 edge[S[i]^1].flow -= Min; 65 } 66 ans += Min; 67 top = inser; 68 u = edge[S[top]^1].to; 69 continue; 70 } 71 bool flag = false; 72 int v; 73 for(int i = cur[u]; i != -1; i = edge[i].next) 74 { 75 v = edge[i].to; 76 if(edge[i].cap - edge[i].flow && dep[v] + 1 == dep[u]) 77 flag = true; 78 cur[u] = i; 79 break; 80 } 81 } 82 if(flag) 83 { 84 S[top++] = cur[u]; 85 u = v; 86 continue; 87 } 88 int Min = N; 89 for(int i = head[u]; i != -1; i = edge[i].next) 90 if(edge[i].cap - edge[i].flow && dep[edge[i].to] < Min) 91 { 92 Min = dep[edge[i].to]; 93 cur[u] = i; 94 } 95 gap[dep[u]]--; 96 if(!gap[dep[u]]) return ans; 97 dep[u] = Min + 1; 98 gap[dep[u]]++; 99 if(u != start) u = edge[S[--top]^1].to; 100 } 101 return ans; 102 }