(強連通+dp)
題意:給定一個單向邊的圖
思路:由於每條路可以走很多遍,那麼最好情況下是一直將每這條路重複走,直至該路上寶石被撿完。於是想到只有圖中出現環可以這樣,於是想到了tarjan縮點。縮點後,對於強連通內部的點來說,最關鍵的是想到:無論從哪一點出發,終點在哪裏,在內部能撿到的寶石數目都是相同的,然後對於每個強連通計數即可;而對於強連通外部的DAG上,利用dp或者記憶化搜索的方式求解全局最優就好。
時間複雜度:縮點和計數的複雜度均爲
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <cstring>
#include <stack>
#define LL long long
using namespace std;
const int maxn = 1000050;
const int inf = 0x3f3f3f3f;
// basic structure
int head[maxn], cnt;
struct edge{
int to, next, w;
}edge[maxn];
void add_edge(int u, int v, int w) {
edge[cnt].to = v; edge[cnt].w = w;
edge[cnt].next = head[u]; head[u] = cnt ++;
}
// old_graph
stack<int> s;
int scc, dfs_clock;
int DFN[maxn], low[maxn], blg[maxn]; //Depth First Number, low, belong
bool instack[maxn]; //vis
// new_graph
LL dp[maxn], dis[maxn];
int new_edge[maxn][3], new_cnt;
void init_graph() {
cnt = 0;
memset(head, -1, sizeof(head));
}
void init_scc(int n) {
scc = dfs_clock = 0;
memset(DFN, -1, sizeof(DFN));
memset(instack, 0, sizeof(instack));
while(!s.empty()) s.pop();
}
void tarjan(int u) {
int v = 0;
DFN[u] = low[u] = dfs_clock ++;
s.push(u);
instack[u] = 1;
for(int k=head[u]; k!=-1; k=edge[k].next) {
v = edge[k].to;
if(DFN[v] == -1)
tarjan(v),low[u] = min(low[u],low[v]);
else if(instack[v])
low[u] = min(low[u],DFN[v]);
}
if(low[u] == DFN[u]) {
scc ++;
do {
v = s.top(); s.pop();
blg[v] = scc;
instack[v] = 0;
}
while(v != u);
}
}
void rebuild_graph(int n) {
new_cnt = 0;
for(int u=1; u<=n; u++) {
for(int k=head[u]; k!=-1; k=edge[k].next) {
int v = edge[k].to, w = edge[k].w;
if(blg[u] == blg[v]) {
int t = floor((-1+sqrt(1+8*w))/2); // n
dp[blg[u]] += (LL)(t+1)*w - (LL)t*(t+1)*(t+2)/6;
}
else {
new_edge[new_cnt][0] = blg[u], new_edge[new_cnt][1] = blg[v];
new_edge[new_cnt][2] = w; new_cnt ++;
}
}
}
init_graph();
for(int i=0; i<new_cnt; i++)
add_edge(new_edge[i][0], new_edge[i][1], new_edge[i][2]);
}
LL dfs(int u) {
if(dis[u] != -1) return dis[u];
LL ret = dp[u];
for(int k=head[u]; k!=-1; k=edge[k].next) {
int v = edge[k].to, w = edge[k].w;
ret = max(ret, dp[u] + (LL)w + dfs(v));
}
dis[u] = ret;
return ret;
}
int main() {
//freopen("test.txt","r",stdin);
int n, m, start;
scanf("%d%d",&n,&m);
init_graph();
init_scc(n);
while(m --) {
int u, v, w;
scanf("%d%d%d",&u,&v,&w);
add_edge(u, v, w);
}
scanf("%d",&start);
for(int i=1; i<=n; i++)
if(DFN[i] == -1) tarjan(i);
rebuild_graph(n);
for(int i=1; i<=scc; i++)
dis[i] = -1;
LL ans = dfs(blg[start]);
printf("%I64d\n",ans);
return 0;
}