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

    public int calculate(String s) {
        if (s == null || s.length() == 0) return 0;
        Stack<Integer> stack = new Stack<Integer>();
        int number = 0, sign = '+';
        for (int i = 0; i < s.length(); i++) {
            char letter = s.charAt(i);
            if (Character.isDigit(letter)) number = number * 10 + letter - '0';
            if ((!Character.isDigit(letter) && letter != ' ') || (i == s.length() - 1)) {
                if (sign == '+') stack.push(number);
                else if (sign == '-') stack.push(-number);
                else if (sign == '*') stack.push(stack.pop() * number);
                else if (sign == '/') stack.push(stack.pop() / number);
                sign = letter;
                number = 0;
            }
        }
        int result = 0;
        for (Integer value : stack) {
            result += value;
        }
        return result;   
    }