RMQ & LCA 专题

poj 1330  裸LCA

#define M 10005

int parent[M]; // 根节点

int Find(int i){
    int j;
    for(j = i; j != parent[j]; j = parent[j]);
    while(i != j){
        int temp = parent[i];
        parent[i] = j; i = temp;
    }
    return j;
}

void Union(int a, int b){
     int x = Find(a), y = Find(b);
     if(x == y) return;
     parent[y]=x;
}

void Make(int SIZE){
     for(int i=0; i<=SIZE; i++) {
         parent[i]=i;
     }
}


struct edge{
    int v,next;
}E[M];

int head[M], p;
int vis[M], rt;
int n, a, b, ans;

void ins(int u, int v) {
    E[p].v = v;     E[p].next = head[u];
    head[u] = p++;
}

bool f;
void dfs(int u){
    if(f) return ;
    vis[u] = 1;
    for(int i = head[u]; i != -1; i = E[i].next){
        int v = E[i].v;
        dfs(v);
        Union(u, v);
    }


    if(u==a && vis[b]){
        ans = Find(b);
        f = true; return;
    }else if(u==b && vis[a]){
        ans = Find(a);
        f = true; return;
    }
}


int main(){
    #ifndef ONLINE_JUDGE
    freopen("in.txt","r",stdin);
    //freopen("out.txt","w",stdout);
    #endif

    int T;
    scanf("%d", &T);
    while(T--){
        memset(head, -1, sizeof head);
        memset(vis, 0, sizeof vis);
        p = 0;
        scanf("%d", &n);
        FOR(i, 1, n){
            scanf("%d%d", &a, &b);
            ins(a, b);
            vis[b] = 1;//b不是根
        }
        FOE(i, 1, n) if(vis[i]==0) {rt = i; break;}  //确定根

        scanf("%d%d", &a, &b);
        f = false;
        memset(vis, 0, sizeof vis);
        Make(n);
        dfs(rt);
        printf("%d\n", ans);
    }

    return 0;
}