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 };