UVa11324TheLargestClique(tarjan)

题目链接：https://onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&category=0&problem=2299&mosmsg=Submission+received+with+ID+26582032

缩点以后求最长路，拓扑和记搜都可以

一定要注意所开数组的大小，边的数组不要开成点的，因为数组开小 WA 了一个小时，实在不行就把数组使劲往大开

#include
using namespace std;
typedef long long ll;
const int maxn = 1010;
int T, n, m;
int u[50010], v[50010], in[maxn];
vector G[maxn];
int h[maxn], cnt = 0;
struct E{
int to, next;
}e[50010];
void add(int u, int v){
e[++cnt].to = v;
e[cnt].next = h[u];
h[u] = cnt;
}
int low[maxn], st[maxn], sccno[maxn], sz[maxn], dfn = 0, scc_cnt = 0;
stack S;
void dfs(int u){
low[u] = st[u] = ++dfn;
S.push(u);

for(int i = h[u] ; i != -1 ; i = e[i].next){
int v = e[i].to;
if(!st[v]){
dfs(v);
low[u] = min(low[u], low[v]);
} else if(!sccno[v]){
low[u] = min(low[u], st[v]);
}
}

if(low[u] == st[u]){
++scc_cnt;
for(;;){
int x = S.top(); S.pop();
sccno[x] = scc_cnt;
++sz[scc_cnt];
if(x == u) break;
}
}
}
void tarjan(){
memset(low, 0, sizeof(low));
memset(st, 0, sizeof(st));
memset(sccno, 0, sizeof(sccno));
dfn = 0, scc_cnt = 0;

for(int i = 1 ; i <= n ; ++i){
if(!st[i]) dfs(i);
}
}
int dp[maxn];
vector path;
void topo(){
queue q;
path.clear(); memset(dp, 0, sizeof(dp));

for(int i = 1 ; i <= scc_cnt ; ++i){
if(!in[i]){
q.push(i);
dp[i] = sz[i];
path.push_back(i);
}
}

while(!q.empty()){
int u = q.front(); q.pop();
for(int i = h[u] ; i != -1 ; i = e[i].next){
int v = e[i].to;
--in[v];
if(!in[v]){
q.push(v);
path.push_back(v);
}
}
}

for(auto u : path){
for(int i = h[u] ; i != -1 ; i = e[i].next){
int v = e[i].to;
dp[v] = max(dp[v], dp[u] + sz[v]);
}
}
}
int DP(int u){
if(dp[u] >= 0) return dp[u];
dp[u] = sz[u];
for(int i = h[u] ; i != -1 ; i = e[i].next){
int v = e[i].to;
dp[u] = max(dp[u], DP(v) + sz[u]);
}
return dp[u];
}
ll read(){ ll s = 0, f = 1; char ch = getchar(); while(ch <'0' || ch > '9'){ if(ch == '-') f = -1; ch = getchar(); } while(ch >= '0' && ch <= '9'){ s = s * 10 + ch - '0'; ch = getchar(); } return s * f; }
int main(){
scanf("%d", &T);
while(T--){
memset(in, 0, sizeof(in));
memset(h, -1, sizeof(h)); cnt = 0;
memset(sz, 0, sizeof(sz));
scanf("%d%d", &n, &m);

for(int i = 1 ; i <= m ; ++i){
scanf("%d%d", &u[i], &v[i]);
add(u[i], v[i]);
}
memset(sz, 0, sizeof(sz));
tarjan();
memset(h, -1, sizeof(h)); cnt = 0;
for(int i = 1 ; i <= m ; ++i){
if(sccno[u[i]] != sccno[v[i]]){
add(sccno[u[i]], sccno[v[i]]);
++in[sccno[v[i]]];
}
}
int ans = 0;
topo();
for(int i = 1 ; i <= n ; ++i) ans = max(ans, dp[i]);
// memset(dp, -1, sizeof(dp));
// for(int i = 1; i <= scc_cnt; i++) ans = max(ans, DP(i));

printf("%d\n", ans);
}
return 0;
}