leetcode 227: 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.

思路：1) 在遍历字符串时计算乘除；2) 为方便使用stack，将减法转换为加分，使加减法可从右往左计算。

class Solution {
public:
    int calculate(string s) {
        long long num = 0;
        char sign = '+';
        for (size_t i = 0; i < s.size(); i++)
        {
            if (isdigit(s[i])) 
            {
                num = num * 10 + s[i] - '0';
            }

            if (!isdigit(s[i]) && !isspace(s[i]) || i == s.size()-1)
            {
                switch (sign)
                {
                case '+':
                    break;
                case '-':
                    num = -num;
                    break;
                case '*':
                    num = numStack.top() * num;
                    numStack.pop();
                    break;
                case '/':
                    num = numStack.top() / num;
                    numStack.pop();
                    break;
                default:
                    break;
                }
                
                numStack.push(num);
                num  = 0;
                sign = s[i];
            }           
        }

        while (!numStack.empty())
        {
            num += numStack.top();
            numStack.pop();
        }

        return (int)num;
    }

private:
    stack<long long> numStack;
};