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.

class Solution {public:    int calculate(string s) {        long long num = 0;          char sign = '+';          for (int 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; //先算乘除                    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();//把减法化为加法在乘除后运算              numStack.pop();          }            return (int)num;              }private:      stack<long long> numStack;  };