LeetCode 224. Basic Calculator

class Solution {
public:
    string s1[1000005];
    string s2[1000005];
    int s3[1000005];
    int top1;
    int top2;
    int top3;
    int calculate(string s) {
        
        top1=0;top2=0;top3=0;
        string str="";
        for(int i=0;i<s.length();i++)
        {
            if(s[i]>='0'&&s[i]<='9')
            {
                str+=s[i];
                continue;
            }
            
            if(str!="")
            {
                s2[top2++] = str;
                str="";
            }
            if(s[i]==' ') continue;
            
            if(s[i]=='(')
            {
                s1[top1++] = s[i];
            }
            else if(s[i]=='+'||s[i]=='-')
            {
                if(top1==0||s1[top1-1]=="(")
                {
                    s1[top1++]=s[i];
                }
                else
                {
                    s2[top2++]=s1[top1-1];
                    top1--;
                    
                    s1[top1++]=s[i];
                } 
            }
            else if(s[i]==')')
            {
                if(s1[top1-1]!="(")
                {
                    s2[top2++]=s1[top1-1];
                    top1-=2;
                }
                else
                {
                    top1--;
                }
            }
        }
        
        if(str!="")
            s2[top2++]=str;
        
        while(top1!=0)
        {
            s2[top2++]=s1[top1-1];
            top1--;
        }
        
        for(int i=0;i<top2;i++)
        {
            if(s2[i]=="+")
            {
                int x =s3[top3-2]+s3[top3-1];
                top3-=2;
                s3[top3++]=x;
            }
            else if(s2[i]=="-")
            {
                int x =s3[top3-2]-s3[top3-1];
                top3-=2;
                s3[top3++]=x;
            }
            else
            {
                s3[top3++]=atoi(s2[i].c_str());
            }
        }
        
        return s3[0];
        
    }
};