224. Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23
解题思路：信晴神，得永生。

class Solution {
public:
    struct node{
        int num;
        char op;
        bool flag;
    };
    void change(string str){
        double num;
        node temp;
        for(int i=0;i<str.length();){
            if(str[i]==' '){
                i++;
                continue;
            }
            if(isdigit(str[i])){
                temp.flag=true;
                temp.num=str[i++]-'0';
                while(i<str.length()&&isdigit(str[i])){
                    temp.num=temp.num*10+str[i++]-'0';
                }
                q.push(temp);
            }
            else {
                temp.flag=false;
                if(str[i]=='(')
                    temp.op=str[i],s.push(temp);
                else if(str[i]==')'){
                    while(!s.empty()&&(s.top().op!='(')){
                        q.push(s.top());s.pop();
                    }
                    if(!s.empty())s.pop();
                }
                else {
                    while(!s.empty()&&(s.top().op!='(')){
                        q.push(s.top());s.pop();
                    }
                    temp.op=str[i];
                    s.push(temp);
                }
                i++;
            }
        }
        while(!s.empty()){
            q.push(s.top());
            s.pop();
        }
    }
    int calculate(string str) {
        change(str);
       // cout<<q.size()<<endl;
        if(q.size()==1)return q.front().num;
        int temp1,temp2;
        node cur,temp;
        while(!q.empty()){
            cur=q.front();
            q.pop();
            if(cur.flag)s.push(cur);
            else {
                temp2=s.top().num;
                s.pop();
                temp1=s.top().num;
                s.pop();
                temp.flag=true;
                if(cur.op=='+')temp.num=temp1+temp2;
                else if(cur.op=='-')temp.num=temp1-temp2;
              //  cout<<temp.num<<endl;
                s.push(temp);
            }
        }
        return s.top().num;
    }
private:
    stack<node>s;
    queue<node>q;
};