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

 

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

 1 class Solution {

 2 public:

 3     int calculate(string s) {

 4         int n=s.length();

 5         if(n<1)

 6             return -1;

 7         stack<int> num;

 8         stack<char> symbol;

 9         symbol.push('+');

10         int i;

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

12         {

13             if(s[i]=='+'||s[i]=='-'||s[i]=='(')

14             {

15                 symbol.push(s[i]);

16                 i++;

17             }

18             else if(s[i]==' ') i++;

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

20                  {

21                      i++;

22                      int right=0;

23                      int tmp=num.top();

24                      num.pop();

25                      char sym=symbol.top();

26                      symbol.pop();

27                      while(sym!='(')

28                      {

29                          

30                          if(sym=='-') tmp=-tmp;

31                          right+=tmp;

32                          tmp=num.top();

33                          num.pop();

34                          sym=symbol.top();

35                          symbol.pop();

36                      }

37                      right+=tmp;

38                      num.push(right);

39                  }

40                  else

41                  {

42                      int right=0;

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

44                      {

45                          right=right*10+s[i]-'0';

46                          i++;

47                      }

48                      num.push(right);

49                  }

50         }

51         int res=0;

52         int tmp_r;

53         char sym_r;

54         while(!symbol.empty())

55         {

56             tmp_r=num.top();

57             sym_r=symbol.top();

58             num.pop();

59             symbol.pop();

60             if(sym_r=='-') tmp_r=-tmp_r;

61             res+=tmp_r;

62         }

63         return res;

64     }

65 };