LeetCode 736. Parse Lisp Expression

class Solution {
public:
    deque<unordered_map<string,int>> scopes;
    
    int evaluate(string expression) {
        int pos=0;
        return eval(expression, pos);
    }
    
    int eval(const string &s, int &pos){
        scopes.push_front(unordered_map<string,int>());
        int value=0; //return val of current expr
        if (s[pos]==‘(‘) ++pos;
        
        string token=getToken(s,pos);
        //cout << token << endl;
        
        if (token=="add"){ // expecting " e1 e2)"
            int e1=eval(s,++pos); //skip ‘ ‘
            int e2=eval(s,++pos);
            value = e1 + e2;
        }else if (token=="mult"){
            int e1=eval(s,++pos); //skip ‘ ‘
            int e2=eval(s,++pos);
            value = e1 * e2;
        }else if (token=="let"){
            // expecting " let v1 e1 v2 e2 ... vn en expr) ...."
            while (pos<s.size()){
                // since ei will be dealt with vi, here can only be vi or expr
                if (s[++pos]==‘(‘){ // if expr is an expression (...)
                    value = eval(s,++pos);
                    break;
                }
                 
                // var can be vi or expr (expr of variable or int)
                string var=getToken(s,pos);
                
                if (s[pos]==‘)‘){ // var is expr
                    if (isalpha(var[0])) // expr is variable
                        value = getValue(var);
                    else // expr is int
                        value = stoi(var);
                    break;
                }
                
                // var is vi, now read next token to be vi
                scopes.front()[var] = eval(s,++pos);
                //cout << var << "<-" << scopes.front()[var] <<endl;
            }
        }else if (isalpha(token[0])){ // variable
            value = getValue(token);
        }else{ // number
            value = stoi(token);
        }
        
        if (s[pos]==‘)‘) ++pos;
        scopes.pop_front();
        return value;
    }

    string getToken(const string &s, int &pos){
        string token="";
        while (pos<s.size()){
            if (s[pos]==‘)‘ || s[pos]==‘ ‘) break;
            token += s[pos++];
        }
        return token;
    }
    
    int getValue(string var){
        for (auto scope:scopes){
            if (scope.count(var)) return scope[var];
        }
        return 0;
    }
};