227. Basic Calculator II(unsolved)

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.

class Solution {
public:
int calculate(string s) {
int result=0,num=0;
char sign='+';
stack<int> st;
for(int i=0;i<s.size() ;i++){
if(isdigit(s[i])){
num=num*10+s[i]-'0';
}
if((!isdigit(s[i])&&s[i]!=' ' )|| i == s.size() - 1){
if(sign=='+') st.push(num);
else if(sign=='-') st.push(-num);
else if(sign=='*'){
int tmp=st.top()*num;
st.pop();
st.push(tmp);
}
else if(sign=='/'){
int tmp=st.top()/num;
st.pop();
st.push(tmp);
}
num=0;
sign=s[i];
}
}
while(!st.empty()){
result+=st.top();
st.pop();
}
return result;
}
};