LeetCode 8 String to Integer (atoi)

class Solution {
public:
    int myAtoi(string str) {
        
        int len = str.length();
        int tag=0;
        int tag2=0;
        char tag3='x';
        string num="";
        for(int i=0;i<len;i++)
        {
            if(str[i]!=' '&&isNumber(str[i])==0)
            {
                if(tag==0)
                    return 0;
                if(tag==1)
                    break;
            }
            if(str[i]==' ')
            {
                if(tag==0)
                    continue;
                if(tag==1)
                    break;
            }

            if(isNumber(str[i])==1)
            {
                tag=1;
                if(str[i]=='+'||str[i]=='-')
                {
                    if(num==""&&tag3!='x')
                        return 0;
                    if(tag2==1&&num!="")
                        break;
                    tag3 = str[i];
                }
                if(str[i]>='0'&&str[i]<='9')
                {
                    tag2 = 1;
                    num+=str[i];
                }
            }
        }
        long long int z = (long long int) pow(2,31)-1;
        long long int y = (z+1)*-1;
        long long int ans=0;
        int pos=0;
        for(int i=0;i<num.length();i++)
        {
            if(num[i]=='0'&&pos==0)
            {
                continue;
            }
            if(num[i]!='0'&&pos==0)
            {
                pos=1;
                ans=ans*10+(num[i]-'0');
                continue;
            }
            if(pos==1)
            {
                ans=ans*10+(num[i]-'0');
            }
            if(ans >= z)
                break;
            
        }
        
    
        
        if(tag3=='-')
            ans=ans*-1; 
        
        if(ans > z)
        {
            ans = z;   
        }
        if(ans < y)
        {
            ans = y;
        }
        return ans;
        
    }
    
    int isNumber(char s)
    {
        if(s>='0'&&s<='9')
            return 1;
        if(s=='+'||s=='-')
            return 1;
        
        return 0;
    }
};