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.

Example 1:
Input: "3+2*2"
Output: 7
Example 2:
Input: " 3/2 "
Output: 1
Example 3:
Input: " 3+5 / 2 "
Output: 5

Note: You may assume that the given expression is always valid. Do not use the eval built-in library function.

在看到数字后的运算符后再根据数字前的运算符来处理该数字
class Solution {
    public int calculate(String s) {
        Stack<Integer> stack = new Stack<>();
        int num = 0;
        char sign = '+';
        for(int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if(Character.isDigit(c)) {
                num = num * 10 + (c - '0');
            }

            // i == s.length() - 1 是为了能处理到算式最后一个数
            // 并且下面的if不能用else if 就是因为算式最后一个是数字时候还得用下面的来接着处理
            if(c == '+' || c == '-' || c == '*' || c == '/' || i == s.length() - 1) {
                if(sign == '+') {
                    stack.push(num);
                } else if(sign == '-') {
                    stack.push(-num);
                } else if(sign == '*') {
                    stack.push(stack.pop() * num);
                } else if(sign == '/') {
                    stack.push(stack.pop() / num);
                }
                num = 0;
                sign = c;
            }
        }
        int res = 0;
        while(!stack.isEmpty()) res += stack.pop();
        return res;
    }
}