和上一道题一样，可以用DAG上的动态规划来做，也可以建立一个源点，用spfa来做
1 #include<cstdio> 2 #include<cstring> 3 #include<iostream> 4 #include<algorithm> 5 #include<stack> 6 #include<vector> 7 using namespace std; 8 9 const int maxn = 5005; 10 int n,m; 11 int first[maxn]; 12 int sc[maxn],scn[maxn],low[maxn],pre[maxn]; 13 int scnt,ecnt,dfs_clock; 14 int dp[maxn]; 15 16 int first1[maxn]; 17 int ecnt1; 18 19 int a[maxn]; 20 int in[maxn]; 21 22 struct Edge{ 23 int v,next; 24 }e[maxn*10]; 25 26 Edge e1[maxn*10]; 27 28 stack<int> S; 29 30 void init(){ 31 ecnt = ecnt1 = 0; 32 memset(first,-1,sizeof(first)); 33 memset(first1,-1,sizeof(first1)); 34 memset(dp,0,sizeof(dp)); 35 } 36 37 void addedges(int u,int v){ 38 e[ecnt].v = v; 39 e[ecnt].next = first[u]; 40 first[u] = ecnt++; 41 } 42 43 void addedges1(int u,int v){ 44 e1[ecnt1].v = v; 45 e1[ecnt1].next = first1[u]; 46 first1[u] = ecnt1++; 47 } 48 49 void dfs(int u){ 50 low[u] = pre[u] = ++dfs_clock; 51 S.push(u); 52 for(int i = first[u];~i;i = e[i].next){ 53 int v = e[i].v; 54 if(!pre[v]){ 55 dfs(v); 56 low[u] = min(low[u],low[v]); 57 } 58 else if(!sc[v]) low[u] = min(low[u],pre[v]); 59 } 60 if(pre[u] == low[u]){ 61 scnt++; 62 for(;;){ 63 int x = S.top();S.pop(); 64 sc[x] = scnt; 65 scn[scnt]+= a[x]; 66 if(x == u) break; 67 } 68 } 69 } 70 71 void find_scc(){ 72 while(!S.empty()) S.pop(); 73 scnt = dfs_clock = 0; 74 memset(low,0,sizeof(low));memset(pre,0,sizeof(pre)); 75 memset(sc,0,sizeof(sc));memset(scn,0,sizeof(scn)); 76 77 for(int i = 1;i <= n;i++) if(!pre[i]) dfs(i); 78 } 79 80 int solve(int p){ 81 if(dp[p]) return dp[p]; 82 for(int i = first1[p];~i;i = e1[i].next){ 83 int v = e1[i].v; 84 dp[p] = max(dp[p],solve(v)); 85 } 86 return dp[p] = dp[p] + scn[p]; 87 } 88 89 int main(){ 90 int T; 91 while(scanf("%d %d",&n,&m) != EOF){ 92 init(); 93 for(int i = 1;i <= n;i++) { 94 scanf("%d",&a[i]); 95 if(a[i] < 0) a[i] = 0; 96 } 97 for(int i = 0;i < m;i++ ){ 98 int u,v; 99 scanf("%d %d",&u,&v);u++;v++; 100 addedges(u,v); 101 } 102 find_scc(); 103 104 memset(in,0,sizeof(in)); 105 for(int u = 1;u <= n;u++){ 106 for(int i = first[u];~i;i = e[i].next){ 107 int v = e[i].v; 108 if(sc[u] != sc[v]) addedges1(sc[u],sc[v]),in[sc[v]]++; 109 } 110 } 111 112 int ans = 0; 113 for(int i = 1;i <= scnt;i++) { 114 ans = max(ans,solve(i)); 115 } 116 printf("%d ",ans); 117 } 118 return 0; 119 }
1 #include<cstdio> 2 #include<cstring> 3 #include<iostream> 4 #include<algorithm> 5 #include<stack> 6 #include<vector> 7 #include<queue> 8 using namespace std; 9 10 const int maxn = 5005; 11 const int INF = 1000000005; 12 int n,m; 13 int first[maxn]; 14 int sc[maxn],scn[maxn],low[maxn],pre[maxn]; 15 int scnt,ecnt,dfs_clock; 16 int dp[maxn]; 17 18 int du[maxn]; 19 int dis[maxn]; 20 int inq[maxn]; 21 22 int first1[maxn]; 23 int ecnt1; 24 25 struct Edge{ 26 int v,next; 27 }e[maxn*10]; 28 29 Edge e1[maxn*10]; 30 31 stack<int> S; 32 vector<int> g[maxn]; 33 int val[maxn],a[maxn]; 34 35 void init(){ 36 ecnt = ecnt1 = 0; 37 memset(first,-1,sizeof(first)); 38 memset(val,0,sizeof(val)); 39 memset(du,0,sizeof(du)); 40 } 41 42 void addedges(int u,int v){ 43 e[ecnt].v = v; 44 e[ecnt].next = first[u]; 45 first[u] = ecnt++; 46 } 47 48 void dfs(int u){ 49 low[u] = pre[u] = ++dfs_clock; 50 S.push(u); 51 for(int i = first[u];~i;i = e[i].next){ 52 int v = e[i].v; 53 if(!pre[v]){ 54 dfs(v); 55 low[u] = min(low[u],low[v]); 56 } 57 else if(!sc[v]) low[u] = min(low[u],pre[v]); 58 } 59 if(pre[u] == low[u]){ 60 scnt++; 61 for(;;){ 62 int x = S.top();S.pop(); 63 sc[x] = scnt; 64 val[scnt] += a[x]; 65 if(x == u) break; 66 } 67 } 68 } 69 70 void find_scc(){ 71 while(!S.empty()) S.pop(); 72 scnt = dfs_clock = 0; 73 memset(low,0,sizeof(low));memset(pre,0,sizeof(pre)); 74 memset(sc,0,sizeof(sc));memset(scn,0,sizeof(scn)); 75 76 for(int i = 1;i <= n;i++) if(!pre[i]) dfs(i); 77 } 78 79 80 int spfa(){ 81 memset(inq, 0, sizeof(inq)); 82 queue<int>q; 83 g[0].clear(); 84 q.push(0); 85 dis[0] = 0; val[0] = 0; 86 for(int i = 1; i <= scnt; i++){if(du[i] == 0)g[0].push_back(i); dis[i] = -INF;} 87 int ans = 0; 88 while(!q.empty()){ 89 int u = q.front(); q.pop(); inq[u] = 0; 90 for(int i = 0; i < g[u].size(); i++){ 91 int v = g[u][i]; 92 if(dis[v] < dis[u] + val[v]){ 93 dis[v] = dis[u] + val[v]; 94 ans = max(ans, dis[v]); 95 if(inq[v] == 0)inq[v] = 1, q.push(v); 96 } 97 } 98 } 99 return ans; 100 } 101 102 int main(){ 103 while(scanf("%d %d",&n,&m) != EOF){ 104 init(); 105 for(int i = 1;i <= n;i++) { 106 scanf("%d",&a[i]); 107 if(a[i] < 0) a[i] = 0; 108 } 109 for(int i = 0;i < m;i++ ){ 110 int u,v; 111 scanf("%d %d",&u,&v);u++;v++; 112 addedges(u,v); 113 } 114 find_scc(); 115 for(int i = 1;i <= scnt;i++) g[i].clear(); 116 117 for(int u = 1;u <= n;u++){ 118 for(int i = first[u];~i;i = e[i].next){ 119 int v = e[i].v; 120 if(sc[u] != sc[v]) g[sc[u]].push_back(sc[v]),du[sc[v]]++; 121 } 122 } 123 printf("%d ",spfa()); 124 } 125 return 0; 126 }