224 Basic Calculator

链接：https://leetcode.com/problems/basic-calculator/
问题描述：
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
Note: Do not use the eval built-in library function.

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计算表达式，这一题是Basic Calculator II https://leetcode.com/problems/basic-calculator-ii/的简化版。

class Solution {public:    int calculate(string s) {        stack<int> num;        stack<char> oper;        string expression="";        for(int i=0;i<s.size();i++)            {if(s[i]!=' ')            expression+=s[i];        }        int temp=0;        for(int i=0;i<expression.length();i++)        {           if(expression[i]-'0'<10&&expression[i]-'0'>-1)                temp=temp*10+expression[i]-'0';            else if(expression[i]=='(')              oper.push(expression[i]);           else            {               if(expression[i]=='*'||expression[i]=='/')               {                       if(!oper.empty()&&(oper.top()=='*'||oper.top()=='/'))                 {                   switch(oper.top())                     {                     case '*':temp=num.top()*temp;num.pop();oper.pop();break;                     case '/':temp=num.top()/temp;num.pop();oper.pop();break;                     default: break;                     }                 }                  num.push(temp);                  temp=0;                  oper.push(expression[i]);                }                else if(expression[i]=='+'||expression[i]=='-')             {                   while(!oper.empty()&&(oper.top()=='+'||oper.top()=='-'||oper.top()=='*'||oper.top()=='/'))                 {                   switch(oper.top())                     {                      case '*':temp=num.top()*temp;num.pop();oper.pop();break;                     case '/':temp=num.top()/temp;num.pop();oper.pop();break;                     case '+':temp=temp+num.top();num.pop();oper.pop();break;                     case '-':temp=num.top()-temp;num.pop();oper.pop();break;                     default: break;                     }                  }                 num.push(temp);                 temp=0;                 oper.push(expression[i]);               }                else if(expression[i]==')')              {                 while(oper.top()!='(')                 {                     switch(oper.top())                     {                     case '+':temp=temp+num.top();num.pop();break;                     case '-':temp=num.top()-temp;num.pop();break;                     case '*':temp=num.top()*temp;num.pop();break;                     case '/':temp=num.top()/temp;num.pop();break;                     default: break;                     }                     oper.pop();                 }                 oper.pop();             }           }        }        while(!oper.empty())        {              switch(oper.top())            {                case '+':temp=temp+num.top();num.pop();break;                case '-':temp=num.top()-temp;num.pop();break;                case '*':temp=num.top()*temp;num.pop();break;                case '/':temp=num.top()/temp;num.pop();break;                default: break;            }        oper.pop();        }        return temp;    }};