B - identify floating point constant problems

When doing this topic WA A lot of time , Finally, all the schemes that are not allowed are summarized
Cases that are not allowed ：
1. Separate integer types . example ：1
2. If the first character of the input string is not ‘+’ or ‘-’. example ：/1.0
3. Integer or floating-point type followed by （‘e’ or ‘E’） Or other characters . example ：1.0e perhaps 1.0/
4.‘e’ or ‘E’ Only connect to the back （‘+’ or ‘-’） Or other characters . example ：1.0e+ perhaps 1.0e/
5.‘+’ or ‘-’ Followed by "0-9" String other than . example ：1.0e+/
matters needing attention ：
‘e’ or ‘E’ hinder ‘+’ It can be omitted . example ：1.0e10

#include<bits/stdc++.h>
using namespace std;
string s;
int p;
int is_int()
{
    
	int flag=0;
	if(s[p]>='0'&&s[p]<='9')
	{
    
		p++;
		flag=1;
		while(1)
		{
    
			if(s[p]>='0'&&s[p]<='9')
				p++;
			else
				break;
		}
	}
	return flag;
}
int is_float()
{
    
	int flag=0;
	if(is_int())
	{
    
		flag=1;
		if(s[p]=='.')
		{
    
			p++;
			if(is_int())
			    flag=2;	
			else
			flag=0;
		}
	}
	return flag;
}
int main()
{
    
	int len;
	while(cin>>s)
	{
    
		len = s.length();
        p=0;
		if(s[p]==' ')p++;
		if(s[p]=='+'||s[p]=='-')p++;
		int flag;
		if(flag=is_float())
		{
    
			if(flag==1&&p==len)
				cout<<"NO"<<endl;
			else if(flag==2&&p==len)
				cout<<"YES"<<endl;
			else
			{
    
					if(s[p]=='e'||s[p]=='E')
					{
    
						p++;
						if(s[p]=='+'||s[p]=='-')
						{
    
							p++;	
						}
						if(is_int())
							{
    
								if(p==len)
								cout<<"YES"<<endl;
								else
								cout<<"NO"<<endl;
							}
							else
							{
    
								cout<<"NO"<<endl;
							}
					}
					else
					{
    
						cout<<"NO"<<endl;
					}
				}
		}
		else
		{
    
			cout<<"NO"<<endl;
		}
	}
}

Thank you for your ideas ：https://blog.csdn.net/godsight/article/details/53219109