强烈推荐这篇文章，一下子就看懂AC了。
#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;
typedef long long ll;
struct Edge{
int too, nxt;
}edge[200005];
int n, m, r, p, uu, vv, ww, tt, w[100005], hea[100005], ecnt, dep[100005];
int fa[100005], siz[100005], son[100005], idx[100005], cnt, wt[100005];
int top[100005];
void add_edge(int fro, int too){
edge[++ecnt].nxt = hea[fro];
edge[ecnt].too = too;
hea[fro] = ecnt;
}
void dfs1(int x, int f){
dep[x] = dep[f] + 1;
fa[x] = f;
siz[x] = 1;
int maxSon=-1;
for(int i=hea[x]; i; i=edge[i].nxt){
int t=edge[i].too;
if(t==f) continue;
dfs1(t, x);
siz[x] += siz[t];
if(siz[t]>maxSon){
son[x] = t;
maxSon = siz[t];
}
}
}
void dfs2(int x, int topf){
idx[x] = ++cnt;
wt[cnt] = w[x] % p;
top[x] = topf;
if(!son[x]) return ;
dfs2(son[x], topf);
for(int i=hea[x]; i; i=edge[i].nxt){
int t=edge[i].too;
if(t==fa[x] || t==son[x]) continue;
dfs2(t, t);
}
}
struct SFT{
int sum[400005];
int tag[400005];
void build(int o, int l, int r){
if(l==r) sum[o] = wt[l];
else{
int mid=(l+r)>>1;
int lson=o<<1;
int rson=lson|1;
if(l<=mid) build(lson, l, mid);
if(mid<r) build(rson, mid+1, r);
sum[o] = (sum[lson] + sum[rson]) % p;
}
}
void pushdown(int o, int l, int r, int lson, int rson, int mid){
tag[lson] = (tag[lson] + tag[o]) % p;
tag[rson] = (tag[rson] + tag[o]) % p;
sum[lson] = (sum[lson] + (mid-l+1) * tag[o] % p) % p;
sum[rson] = (sum[rson] + (r-mid) * tag[o] % p) % p;
tag[o] = 0;
}
void update(int o, int l, int r, int x, int y, int k){
if(l>=x && r<=y){
sum[o] = (sum[o] + (r-l+1) * k % p) % p;
tag[o] = (tag[o] + k) % p;
}
else{
int mid=(l+r)>>1;
int lson=o<<1;
int rson=lson|1;
if(tag[o]) pushdown(o, l, r, lson, rson, mid);
if(x<=mid) update(lson, l, mid, x, y, k);
if(mid<y) update(rson, mid+1, r, x, y, k);
sum[o] = (sum[lson] + sum[rson]) % p;
}
}
int query(int o, int l, int r, int x, int y){
if(l>=x && r<=y) return sum[o];
else{
int mid=(l+r)>>1;
int lson=o<<1;
int rson=lson|1;
int ans=0;
if(tag[o]) pushdown(o, l, r, lson, rson, mid);
if(x<=mid) ans = (ans + query(lson, l, mid, x, y)) % p;
if(mid<y) ans = (ans + query(rson, mid+1, r, x, y)) % p;
return ans;
}
}
}sgt;
void updRange(int uu, int vv, int ww){
ww %= p;
while(top[uu]!=top[vv]){
if(dep[top[uu]]<dep[top[vv]]) swap(uu, vv);
sgt.update(1, 1, n, idx[top[uu]], idx[uu], ww);
uu = fa[top[uu]];
}
if(dep[uu]>dep[vv]) swap(uu, vv);
sgt.update(1, 1, n, idx[uu], idx[vv], ww);
}
int queryRange(int uu, int vv){
int ans=0;
while(top[uu]!=top[vv]){
if(dep[top[uu]]<dep[top[vv]]) swap(uu, vv);
ans = (ans + sgt.query(1, 1, n, idx[top[uu]], idx[uu])) % p;
uu = fa[top[uu]];
}
if(dep[uu]>dep[vv]) swap(uu, vv);
ans = (ans + sgt.query(1, 1, n, idx[uu], idx[vv])) % p;
return ans;
}
int main(){
cin>>n>>m>>r>>p;
for(int i=1; i<=n; i++)
scanf("%d", &w[i]);
for(int i=1; i<n; i++){
scanf("%d %d", &uu, &vv);
add_edge(uu, vv);
add_edge(vv, uu);
}
dep[r] = 1;
dfs1(r, 0);
dfs2(r, r);
sgt.build(1, 1, n);
while(m--){
scanf("%d", &tt);
if(tt==1){
scanf("%d %d %d", &uu, &vv, &ww);
updRange(uu, vv, ww);
}
else if(tt==2){
scanf("%d %d", &uu, &vv);
printf("%d\\n", queryRange(uu, vv));
}
else if(tt==3){
scanf("%d %d", &uu, &ww);
sgt.update(1, 1, n, idx[uu], idx[uu]+siz[uu]-1, ww);
}
else{
scanf("%d", &uu);
printf("%d\\n", sgt.query(1, 1, n, idx[uu], idx[uu]+siz[uu]-1));
}
}
return 0;
}