Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7

" 3/2 " = 1

" 3+5 / 2 " = 5

 

Note: Do not use the eval built-in library function.

 1 class Solution {

 2 public:

 3     int calculate(string s) {

 4         stack<int> num;

 5         stack<char> symbol;

 6         symbol.push('+');

 7         int n;

 8         n=s.length();

 9         int right;

10         if(n<1) return -1;

11         int i;

12         for(i=0;i<n;)

13         {

14             if(s[i]=='+'||s[i]=='-'||s[i]=='*'||s[i]=='/')

15             {

16                 symbol.push(s[i]);

17                 i++;

18             }

19             else if(s[i]==' ')

20                     i++;

21                  else

22                  {

23                      int right=0;

24                      while(i<n&&s[i]>='0'&&s[i]<='9')

25                      {

26                          right=right*10+(s[i]-'0');

27                          i++;

28                      }

29                      if((!symbol.empty())&&(symbol.top()=='*'||symbol.top()=='/'))

30                      {

31                          right=cal_helper(num.top(),right,symbol.top());

32                          num.pop();

33                          symbol.pop();

34                      }

35                      num.push(right);

36                  }

37         }

38         int res=0;

39         char sym;

40         int left;

41         while(!symbol.empty())

42         {

43             left=num.top();

44             sym=symbol.top();

45             if(sym=='-')

46               left=-left;

47             num.pop();

48             symbol.pop();

49             res+=left;

50         }

51         return res;

52     }

53 private:

54     int cal_helper(int left, int right, char s)

55     {

56         

57         switch(s)

58         {

59         case '*':return left*right;

60         case '/':return left/right;

61         default:return 0;

62         }

63         

64     }

65 };