原代码

#include<cstdio>
#include<cstring>
#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
const int maxn = 200005;
int n;
struct hjtTree {
	char v;
	int ls, rs;
} tree[maxn*20];
int tot, caozuo, tail[maxn];
int rt[maxn];
int jmp[maxn];
char ch;

void upd(int p) {
	if(tree[p].ls==0 && tree[p].rs==0) return;
	tree[p].v = tree[tree[p].ls].v + tree[tree[p].rs].v;
}


int build(int l, int r, char v) {
	int p = ++tot;
	if(l==r) {
		tree[p].v = v;
		return p;
	}
	int midd = (l+r) >> 1;
	tree[p].ls = build(l, midd, v);
	tree[p].rs = build(midd+1, r, v);
	upd(p);
	return p;
}


int insert(int p, int l, int r, int x, char v) {
	int q = ++tot;
	tree[q] = tree[p];
	if(tree[q].ls==tree[q].rs && tree[q].rs==0) {  //----------------------------------
		tree[q].v = v;
		return q;
	}
	int midd = (l+r) >> 1;
	if(x<=midd) {
		tree[q].ls = insert(tree[q].ls, l, midd, x, v);
	}
	else if(x>midd) {
		tree[q].rs = insert(tree[q].rs, midd+1, r, x, v);
	}
	upd(q);
	return q;
}


char query(int p, int l, int r, int x) {
	if(l==r) {
		return tree[p].v;
	}
	int midd = (l+r) >> 1;
	if(x <= midd) return query(tree[p].ls, l, midd, x);
	else return query(tree[p].rs, midd+1, r, x);
}



int main() {
	#ifndef ONLINE_JUDGE
		freopen("input.txt", "r", stdin);
		// freopen("output.txt", "w", stdout);
	#endif
	scanf("%d", &n);
	rt[0] = build(1, n, 0);
	for(int i=1; i<=n; ++i) {
		scanf(" %c", &ch);
		if(ch=='T') {
			scanf(" %c", &ch);
			int p = caozuo;
			while(jmp[p]) p = jmp[p];
			++caozuo;
			
			tail[caozuo] = tail[p] + 1;
			rt[caozuo] = insert(rt[p], 1, n, tail[caozuo], ch);
		}
		else if(ch=='U') {
			int t; 
			scanf("%d", &t);
			++caozuo;
			jmp[caozuo] = caozuo-1-t;
		}
		else {
			int t;
			scanf("%d", &t);
			int p = caozuo;
			while(jmp[p]) p = jmp[p];
			printf("%c\n", query(rt[p], 1, n, t));
		}
	}
	return 0;
}

WA掉1、3、6三个点。

改成

#include<cstdio>
#include<cstring>
#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
const int maxn = 200005;
int n;
struct hjtTree {
	char v;
	int ls, rs;
} tree[maxn*20];
int tot, caozuo, tail[maxn];
int rt[maxn];
int jmp[maxn];
char ch;

void upd(int p) {
	if(tree[p].ls==0 && tree[p].rs==0) return;
	tree[p].v = tree[tree[p].ls].v + tree[tree[p].rs].v;
}


int build(int l, int r, char v) {
	int p = ++tot;
	if(l==r) {
		tree[p].v = v;
		return p;
	}
	int midd = (l+r) >> 1;
	tree[p].ls = build(l, midd, v);
	tree[p].rs = build(midd+1, r, v);
	upd(p);
	return p;
}


int insert(int p, int l, int r, int x, char v) {
	int q = ++tot;
	tree[q] = tree[p];
	if(l==r) {//-------------------------------------------
		tree[q].v = v;
		return q;
	}
	int midd = (l+r) >> 1;
	if(x<=midd) {
		tree[q].ls = insert(tree[q].ls, l, midd, x, v);
	}
	else if(x>midd) {
		tree[q].rs = insert(tree[q].rs, midd+1, r, x, v);
	}
	upd(q);
	return q;
}


char query(int p, int l, int r, int x) {
	if(l==r) {
		return tree[p].v;
	}
	int midd = (l+r) >> 1;
	if(x <= midd) return query(tree[p].ls, l, midd, x);
	else return query(tree[p].rs, midd+1, r, x);
}



int main() {
	#ifndef ONLINE_JUDGE
		freopen("input.txt", "r", stdin);
		// freopen("output.txt", "w", stdout);
	#endif
	scanf("%d", &n);
	rt[0] = build(1, n, 0);
	for(int i=1; i<=n; ++i) {
		scanf(" %c", &ch);
		if(ch=='T') {
			scanf(" %c", &ch);
			int p = caozuo;
			while(jmp[p]) p = jmp[p];
			++caozuo;
			
			tail[caozuo] = tail[p] + 1;
			rt[caozuo] = insert(rt[p], 1, n, tail[caozuo], ch);
		}
		else if(ch=='U') {
			int t; 
			scanf("%d", &t);
			++caozuo;
			jmp[caozuo] = caozuo-1-t;
		}
		else {
			int t;
			scanf("%d", &t);
			int p = caozuo;
			while(jmp[p]) p = jmp[p];
			printf("%c\n", query(rt[p], 1, n, t));
		}
	}
	return 0;
}

AC了。

改动的地方在注释那一行，也就是把判定左右子树都为0改为判定l==r。

求原因QwQ