[Leetcode] Basic Calculator

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-negati ve integers and empty spaces .

Assume that the given expression was 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.

Using the stack, if you encounter an opening parenthesis ' (' press the previous value and the 1.-1 expression operation symbol into the stack, you encounter a closing parenthesis ') ', the operation symbol and value removal are calculated.

1 classSolution {2  Public:3     intCalculatestrings) {4         if(s.length () = =0)return 0;5stack<int>Mystack;6         intresult=0;7         intop=1;8          for(intI=0; I<s.length (); i++)9         {Ten             if(s[i]>='0'&& s[i]<='9') One             { A                 intnumber=s[i]-'0'; -                  while((i+1) <s.length () && s[i+1]>='0'&& s[i+1]<='9') -                 { thei++; -number=number*Ten+s[i]-'0'; -                 } -result+=op*Number ; +             } -             if(s[i]==' ') +             { A                 Continue; at             } -             if(s[i]=='+') -             { -op=1; -             } -             if(s[i]=='-') in             { -op=-1; to             } +             if(s[i]=='(') -             { the Mystack.push (result); *result=0; $ Mystack.push (OP);Panax Notoginsengop=1; -             } the             if(s[i]==')') +             { Aop=mystack.top (); the Mystack.pop (); +result=op*result+mystack.top (); - Mystack.pop (); $op=1; $             } -         } -         returnresult; the     } -};

[Leetcode]basic Calculator