UVA 11324 The Largest Clique

#include<bits/stdc++.h>
using namespace std;

const int maxn = 1005,maxm = 5e5+1;

int head[maxn],to[maxm],nxt[maxm];

void addEdge(int u,int v,int i)
{
    to[i] = v;
    nxt[i] = head[u];
    head[u] = i;
}

int pre[maxn],low[maxn],sccno[maxn],dfs_clock,scc_cnt;
stack<int> stk;

void tarjan(int u)
{
    pre[u] = low[u] = ++dfs_clock;
    stk.push(u);
    for(int i = head[u]; ~i; i = nxt[i]){
        int v = to[i];
        if(!pre[v]){
            tarjan(v);
            low[u] = min(low[u],low[v]);
        }else if(!sccno[v]){
            low[u] = min(low[u],pre[v]);
        }
    }
    if(pre[u] == low[u]){
        scc_cnt++;
        while(stk.size()){
            int x = stk.top(); stk.pop();
            sccno[x] = scc_cnt;
            if(x == u) break;
        }
    }
}

void find_scc(int n)
{
    memset(pre,0,sizeof(pre));
    memset(sccno,0,sizeof(sccno));
    dfs_clock = scc_cnt = 0;
    for(int i = 0; i < n; i++){
        if(!pre[i]) tarjan(i);
    }
}

vector<int> G[maxn];
int wei[maxn], deg[maxn];

void buildDAG(int n)
{
    for(int i = 1; i <= scc_cnt; i++) G[i].clear(),deg[i] = wei[i] = 0;
    for(int u = 0; u < n; u++){
        int v0 = sccno[u]; wei[v0]++;
        for(int i = head[u]; ~i; i = nxt[i]){
            int v1 = sccno[to[i]];
            if(v0 != v1) G[v0].push_back(v1),deg[v1]++;
        }
    }
}

int dp[maxn];
int topo()
{
    memset(dp,-1,sizeof(dp));
    queue<int> q;
    for(int i = 1; i <= scc_cnt; i++){
        if(!deg[i]) q.push(i),dp[i] = wei[i];
    }
    while(q.size()){
        int u = q.front(); q.pop();
        for(int i = 0; i < (int)G[u].size(); i++){
            int v = G[u][i];
            dp[v] = max(dp[v],dp[u]+wei[v]);
            if(--deg[v] == 0) q.push(v);
        }
    }
    int ans = 0;
    for(int i = 1; i <= scc_cnt; i++){
        ans = max(dp[i],ans);
    }
    return ans;
}

int main()
{
    //freopen("in.txt","r",stdin);
    int T; scanf("%d",&T);
    while(T--){
        int n,m;
        memset(head,-1,sizeof(head));
        scanf("%d%d",&n,&m);
        for(int i = 0; i < m; i++){
            int u,v; scanf("%d%d",&u,&v);
            addEdge(u-1,v-1,i);
        }
        find_scc(n);
        buildDAG(n);
        printf("%d\n",topo());
    }
    return 0;
}