P1739 expression bracket matching problem solution

subject

link

https://www.luogu.com.cn/problem/P1739

Literal description

Title Description
Let's say an expression has letters （ A lowercase letter ）、 Operator （+,—,*,/） And left and right little （ round ） Bracket construction , With “@” As the terminator of an expression . Please write a program to check whether the left and right parentheses in the expression match , If the match , Then return to “YES”; Otherwise return to “NO”. The length of the expression is less than 255, The left parenthesis is less than 20 individual .

Input format
a line ： expression

Output format
a line ：“YES” or “NO”

I/o sample
Input #1 Copy
2*(x+y)/(1-x)@
Output #1 Copy
YES
Input #2 Copy
(25+x)(a(a+b+b)@
Output #2 Copy
NO
explain / Tips
The length of the expression is less than 255, The left parenthesis is less than 20 individual

Code implementation

#include<bits/stdc++.h>
using namespace std;

const int maxn=50;
string a;
int top=0,flag=1;
int main(){
    
	cin>>a;
	int lena=a.length();
	for(int i=0;i<lena;i++){
    
		if(a[i]=='(')top++;
		else if(a[i]==')'){
    
			if(top>0)top--;
			else {
    
				flag=0;
				break;
			}
		}
	}
	if(flag){
    
		if(top)cout<<"NO"<<endl;
		else cout<<"YES"<<endl;
	}
	else cout<<"NO"<<endl;
	return 0;
}