[LeetCode] 227. Basic Calculator II

https://leetcode.com/problems/basic-calculator-ii

public class Solution {
    public int calculate(String s) {
        Stack<Integer> stack = new Stack<>();
        char sign = '+';
        int num = 0;
        int result = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) >= '0' && s.charAt(i) <= '9') {
                num = num * 10 + s.charAt(i) - '0';
            } 
            if (!(s.charAt(i) >= '0' && s.charAt(i) <= '9') && s.charAt(i) != ' ' || i == s.length() - 1) {
                if (sign == '+') {
                    stack.push(num);
                } else if (sign == '-') {
                    stack.push(-1 * num);
                } else if (sign == '*') {
                    stack.push(stack.pop() * num);
                } else if (sign == '/') {
                    stack.push(stack.pop() / num);
                }
                num = 0;
                sign = s.charAt(i);
            }
        }
        while (!stack.isEmpty()) {
            result += stack.pop();
        }
        return result;
    }
}

思路参考：http://www.cnblogs.com/grandyang/p/4601208.html