一道裸的网络流求最大流问题
1 //一般增广路，每次不断在生于网络找层次网络，直到找不到说明已找到最大流量 2 #include <cstdio> 3 #include <cstring> 4 #include <queue> 5 #include <iostream> 6 const int N = 210; 7 using namespace std; 8 9 int n,m; 10 int cap[N][N] , flow[N][N] , lev[N]; 11 int pre[N];//回溯标记 12 13 bool find_levelnet() 14 { 15 memset(pre , -1 , sizeof(pre)); 16 queue<int> q; 17 q.push(1); 18 pre[1] = 0; 19 memset(lev , 0x3f , sizeof(lev)); 20 lev[1] = 1; 21 while(!q.empty()){ 22 int u = q.front(); 23 q.pop(); 24 for(int i=1 ; i<=n ; i++){ 25 int tmp = cap[u][i] - flow[u][i]; 26 if(tmp && lev[i] > lev[u]+1){ 27 lev[i] = lev[u]+1; 28 pre[i] = u; 29 q.push(i); 30 } 31 } 32 } 33 if(lev[n] > n) return false; 34 return true; 35 } 36 37 int get_min_flow() 38 { 39 int minn = 0x3f3f3f3f; 40 for(int i = n ; i!=1 ; i=pre[i]){ 41 int u = pre[i]; 42 minn = min(minn , cap[u][i] - flow[u][i]); 43 } 44 return minn; 45 } 46 47 void update(int d) 48 { 49 for(int i = n ; i!=1 ; i=pre[i]){ 50 int u = pre[i]; 51 flow[u][i] += d; 52 flow[i][u] -= d; 53 } 54 } 55 56 int main() 57 { 58 //freopen("a.in","rb",stdin); 59 int u,v,d; 60 while(scanf("%d%d" , &m , &n) == 2){ 61 memset(cap , 0 ,sizeof(cap)); 62 memset(flow , 0 ,sizeof(flow)); 63 for(int i=0 ; i<m ; i++){ 64 scanf("%d%d%d" , &u , &v , &d); 65 cap[u][v] += d; //可能有重边不断叠加 66 } 67 68 int ans = 0; 69 while(find_levelnet()){ 70 int min_flow = get_min_flow(); 71 ans += min_flow; 72 update(min_flow); 73 } 74 75 printf("%d ",ans); 76 } 77 return 0; 78 }