思路是用主席树维护每个版本，为什么没输出？

 #include<bits/stdc++.h>
 #define ll long long
const int MAX=1e5+10;
using namespace std;
inline int read() {
  int fh = 1, res = 0; char ch = getchar();
  for(; !isdigit(ch); ch = getchar()) if(ch == '-') fh = -1;
  for(; isdigit(ch); ch = getchar()) res = (res << 3) + (res << 1) + (ch ^ '0');
  res = res * fh;
  return res;
}
inline void write(ll x) {
    if(x<0){putchar('-');x=-x;}
    if(x>9) write(x/10);
    putchar(x%10+'0');
}
struct node{int lc,rc;char w;}a[MAX<<5];
int tot,root[MAX],js;
int build(int l,int r)
{
	int k=++tot;
	int mid=(l+r)>>1;if(l==r) return k;
	a[k].lc=build(l,mid);a[k].rc=build(mid+1,r);
	return k;
}
int change(int l,int r,int x,char y,int k)
{
	a[++tot]=a[k];k=tot;
	if(l==r){a[k].w=y;return k;}
	int mid=(l+r)>>1;
	if(x<=mid) a[k].lc=change(l,mid,x,y,a[k].lc);
	else a[k].rc=change(mid+1,r,x,y,a[k].rc);
	return k;
}
char ask(int l,int r,int x,int k)
{
//	cout<<l<<" "<<r<<" "<<k<<" "<<a[k].w<<endl;
	if(l==r) return a[k].w;
	int mid=(l+r)>>1;
	if(x<=mid) return ask(l,mid,x,a[k].lc);
	else return ask(mid+1,r,x,a[k].rc);
}
int main()
{
	int n=read();
//	build(1,n); 
	for(int i=1,cx=0;i+cx<=n;i++)
	{
		char opt;scanf(" %c",&opt);root[i]=root[i-1];
		if(opt=='T') {char x;scanf(" %c",&opt);root[i]=change(1,n,++js,x,root[i-1]);}
		if(opt=='U') {int x=read();root[i]=root[i-x];}
		if(opt=='Q') {int x=read();printf("%c\n",ask(1,n,x,root[i-1]));i--;cx++;}
	}
	return 0;
}