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为什么只能对三个点？ 大佬求助！

#include<bits/stdc++.h>
using namespace std;
string s;
int top=0;
string a;
int main(){
	int n;
	cin>>n; 
	for(int j=1;j<=n;j++){
		cin>>s;
		bool flag=true;
		top=0;
		for(int i=0;i<s.length();i++){
			if(s[i]=='{'&&((a[top]=='{')||top==0)){
				a[++top]=s[i];
			}
			else if(s[i]=='['&&((a[top]=='{'||a[top]=='[')||top==0)){
				a[++top]=s[i];
			}
			else if(s[i]=='('&&((a[top]=='{'||a[top]=='['||a[top]=='(')||top==0)){
				a[++top]=s[i];
			}
			else if(s[i]=='<'){
				a[++top]=s[i];
			}
			else if(s[i]==')'&&a[top]=='('){
				top--;
			}
			else if(s[i]==']'&&a[top]=='['){
				top--;
			}
			else if(s[i]=='>'&&a[top]=='<'){
				top--;
			}
			else if(s[i]=='}'&&a[top]=='{'){
				top--;
			}
			else{
				cout<<"NO"<<endl;
				flag=false;
				break;
			}
		}
		if(top==0&&flag){
			cout<<"YES"<<endl;
		}
		else if(flag&&top!=0){
			cout<<"NO"<<endl;
		}
	}
	return 0;
}

AC代码:

#include<iostream>
#include<cstring>
#include<cstring>
using namespace std;
bool judge(string s)
{
    char st[300];
    memset(st,0,sizeof(0));
    int top=-1;
    for(int i=0;i<s.length();i++)
    {
            if(s[i]=='<')st[++top]=s[i];
            if(s[i]=='>'){
                if(st[top]=='<')top--;
                else return 0;
            }
            if(s[i]=='('){
                if(top>-1&&st[top]=='<')return 0;
                else st[++top]=s[i];
            }
            if(s[i]==')'){
                if(st[top]=='(')top--;
                else return 0;
            }
            if(s[i]=='['){
                if(top>-1&&(st[top]=='<'||st[top]=='('))return 0;
                else st[++top]=s[i];
            }
            if(s[i]==']'){
                if(st[top]=='[')top--;
                else return 0;
            }
            if(s[i]=='{'){
                if(top>-1&&(st[top]=='<'||st[top]=='('||st[top]=='['))return 0;
                else st[++top]=s[i];
            }
            if(s[i]=='}'){
                if(st[top]=='{')top--;
                else return 0;
            }
    }
    if(top==-1)return 1;
    else return 0;
}
int main()
{
    int n;
    cin>>n;
    while(n--)
    {
        string s;
        cin>>s;
        if(judge(s))cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}