leetcode 227. Basic Calculator II 字符串表达式计算

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.

这道题就是最经典的字符串表达式的计算问题，可以先做中序转后序，然后去计算，可以参考上一道题leetcode 224. Basic Calculator 字符串表达式计算

代码如下：

import java.util.Stack;public class Solution {    public int calculate(String s)    {        int len;        if(s==null || (len = s.length())==0)             return 0;        Stack<Integer> stack = new Stack<Integer>();        int num = 0;        char sign = '+';        for(int i=0;i<len;i++)        {            if(Character.isDigit(s.charAt(i)))                num = num*10+s.charAt(i)-'0';            if((!Character.isDigit(s.charAt(i)) &&' '!=s.charAt(i)) || i==len-1)            {                if(sign=='-')                    stack.push(-num);                if(sign=='+')                    stack.push(num);                if(sign=='*')                    stack.push(stack.pop()*num);                if(sign=='/')                    stack.push(stack.pop()/num);                sign = s.charAt(i);                num = 0;            }        }        int re = 0;        for(int i:stack)            re += i;        return re;    }}