(medium)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.

代码如下：

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 ret=0;
        for(int i:stack){
            ret+=i;
        }
        return ret;
    }
}

运行结果：