LeetCode227: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:

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

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

和Basic Calculator比較相似。都是很巧妙的解法。

runtime:24ms

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

    int re = 0;
    while(!st.empty()){
        re += st.top();
        st.pop();
    }
    return re;

    }
};