#include<iostream>
#include<ext/pb_ds/assoc_container.hpp>  
#include<ext/pb_ds/tree_policy.hpp>  
using namespace __gnu_pbds;  
using namespace std;  
struct node
{
	int v,id;
	node(int a,int b){v=a;id=b;}
	bool operator >(node b) const
	{return v==b.v?id>b.id:v>b.v;}
};
tree<node,null_type,greater<node>,rb_tree_tag,tree_order_statistics_node_update> T,TE;
int main()
{
	int n,m,k,s=0,ans=0;
	char c;
	scanf("%d%d",&n,&m);
	while(n--)
	{
		cin>>c;
		scanf("%d",&k);
		if(c=='I')
		{
			k+=s;
			if(k>=m) T.insert(node(k,n));
		}
		else if(c=='A') m-=k,s-=k;
		else if(c=='S')
		{
			m+=k,s+=k;
			T.split(node(m,-1),TE);
			ans+=TE.size();
		}
		else if(c=='F')
		{
			if(k>T.size()) cout<<-1<<endl;
			else cout<<T.find_by_order(k-1)->v-s<<endl;
		}
	}
	printf("%d\n",ans);
	return 0;
}

为什么while循环可以

#include<iostream>
#include<ext/pb_ds/assoc_container.hpp>  
#include<ext/pb_ds/tree_policy.hpp>  
using namespace __gnu_pbds;  
using namespace std;  
struct node
{
	int v,id;
	node(int a,int b){v=a;id=b;}
	bool operator >(node b) const
	{return v==b.v?id>b.id:v>b.v;}
};
tree<node,null_type,greater<node>,rb_tree_tag,tree_order_statistics_node_update> T,TE;
int main()
{
	int n,m,k,s=0,ans=0;
	char c;
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++)
	{
		cin>>c;
		scanf("%d",&k);
		if(c=='I')
		{
			k+=s;
			if(k>=m) T.insert(node(k,n));
		}
		else if(c=='A') m-=k,s-=k;
		else if(c=='S')
		{
			m+=k,s+=k;
			T.split(node(m,-1),TE);
			ans+=TE.size();
		}
		else if(c=='F')
		{
			if(k>T.size()) cout<<-1<<endl;
			else cout<<T.find_by_order(k-1)->v-s<<endl;
		}
	}
	printf("%d\n",ans);
	return 0;
}

但for循环不行，难绷