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.

题目含义：实现一个计算器

 1     public int calculate(String s) {
 2         if (s == null || s.length() == 0) return 0;
 3         Stack<Integer> stack = new Stack<Integer>();
 4         int number = 0, sign = '+';
 5         for (int i = 0; i < s.length(); i++) {
 6             char letter = s.charAt(i);
 7             if (Character.isDigit(letter)) number = number * 10 + letter - '0';
 8             if ((!Character.isDigit(letter) && letter != ' ') || (i == s.length() - 1)) {
 9                 if (sign == '+') stack.push(number);
10                 else if (sign == '-') stack.push(-number);
11                 else if (sign == '*') stack.push(stack.pop() * number);
12                 else if (sign == '/') stack.push(stack.pop() / number);
13                 sign = letter;
14                 number = 0;
15             }
16         }
17         int result = 0;
18         for (Integer value : stack) {
19             result += value;
20         }
21         return result;   
22     }

 

posted @ 2017-10-18 13:46  daniel456  阅读(117)  评论(0)  编辑  收藏  举报