求助

#include <bits/stdc++.h>

using namespace std;

int N, M, S;
const int MAXN(5e5 + 5), MAXM(MAXN + MAXN);
int head[MAXN], tot;
int fa[MAXN][30];
int dep[MAXN];
bool book[MAXN];

int rem[MAXN];
struct Arc {
    int to, nex;
}arc[MAXM];

void Add(const int u, const int v) {
    arc[++tot].nex = head[u];
    head[u] = tot;
    arc[tot].to = v;
}

void dfs(const int u, const int v, const int depth) {
    dep[u] = depth;
	    fa[u][0] = v;
	    for (int i(head[u]); i; i = arc[i].nex) {
			const int to(arc[i].to);
			if (to != v) {
			    dfs(to, u, depth + 1);
			}
	}
}

int getFa(int u, int lev) {
    for (int i(25); i >= 0; --i) {
    	const int t(1 << i);
	while (lev >= t) {
	    lev -= t;
	    u = fa[u][i];
	}
    }
    return u;
}

void Init() {
    for (int k(1); k <= 25; ++k) {
	for (int u(1); u <= N; ++u) {
	    int t(u);
	    for (int i(1); i <= 2; ++i) {
		t = fa[t][k - 1];
	    }
	    fa[u][k] = t;
	}
    }
}

int getLCA(int a, int b) {
	if (dep[a] <= dep[b]) {
		swap(a, b);
	}
	a = getFa(a, dep[a] - dep[b]);
	if (a == b) {
		return a;
	}
	for (int i(25); i >0; --i) {
		const int ta(getFa(a, i)), tb(getFa(b, i));
		if (ta != tb) {
			a = ta;
			b = tb;
		}
	}
	return fa[a][0];
}

int main() {
    scanf("%d %d %d", &N, &M, &S);
    for (int i(1); i < N; ++i) {
		int u, v;
		scanf("%d %d", &u, &v);
		Add(u, v); Add(v, u);
    }
    dfs(S, 0, 0);
    Init();
    while (M--) {
	int a, b;
		scanf("%d %d", &a, &b);
		printf("%d\n", getLCA(a,b));
    }
    return 0;
}

wa了最后两个点 求助！