UVALive 4287 Proving Equivalence

#include<bits/stdc++.h>
using namespace std;
const int maxn = 2e4+5;
const int maxm = 5e4+5;

int head[maxn],nxt[maxm],to[maxm],ecnt;
void addEdge(int u,int v)
{
    nxt[ecnt] = head[u];
    to[ecnt] = v;
    head[u] = ecnt++;
}

void initGraph(int n)
{
    memset(head,-1,sizeof(int)*(n+1)); ecnt = 0;
}

int sccno[maxn],pre[maxn],low[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(low[u] == pre[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(int)*(n+1));
    memset(sccno,0,sizeof(int)*(n+1));
    scc_cnt = dfs_clock = 0;
    for(int i = 0; i < n; i++){
        if(!pre[i]) tarjan(i);
    }
}

int ind[maxn],outd[maxn];

int main()
{
    //freopen("in.txt","r",stdin);
    int T; scanf("%d",&T);
    while(T--){
        int n,m; scanf("%d%d",&n,&m);
        initGraph(n);
        for(int i = 0; i < m; i++){
            int u,v; scanf("%d%d",&u,&v);
            addEdge(u-1,v-1);
        }
        find_scc(n);
        if(scc_cnt == 1) {
            printf("0\n"); continue;
        }
        for(int i = 1; i <= scc_cnt; i++) ind[i] = outd[i] = 0;
        for(int u = 0; u < n; u++){
            for(int i = head[u]; ~i; i = nxt[i]){
                int v = to[i];
                if(sccno[u] != sccno[v]) outd[sccno[u]]++,ind[sccno[v]]++;
            }
        }
        int a = 0,b = 0;
        for(int i = 1; i <= scc_cnt; i++){
            if(!outd[i]) b++;
            if(!ind[i]) a++;
        }
        printf("%d\n",max(a,b));
    }
    return 0;
}