【LeetCode】227. Basic Calculator

Problem:

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.

Solution:

字符串分成两种情况：数字num和运算符op，结果设为res，考虑到先乘除后加减的运算法则，设中间数mid优先计算乘除，再与res相加.

*> C++ version

class Solution {
public:
    int calculate(string s) 
    {
       int res=0, num=0, op='+', i=0, mid=0;
       while(i<s.size())
       {
           num = 0;  ////每一轮要初始化num
           //获取一整个数字存储在num中
           if(isdigit(s[i]))
           {
                while(i<s.size()&&isdigit(s[i]))  //连续数字字符作一个数 
                {   
                     num = num*10+(s[i]-'0');
                     i++;
                }
        
               //获取运算符存储在op中
                if(op=='+'||op=='-')
                {
                    res = res+mid;
                    mid = num*(op=='-'?-1:1);
                }     
                else if(op=='/')
                {
                    mid = mid/num;
                }
                else if(op=='*')
                {
                    mid = mid*num;
                }
           }
           else if(s[i]==' ')         //不要漏了开头空格这种情况= - =它喵被摆了一道
                i++;
           else     
           {
                op=s[i];
                i++;
           }
       } 
        res = res+mid;
        return res;
    }
};