Basic Calculator II leetcode

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

这回没有括号, sign记录符号, num记录数字, 每当遇到新的符号时候入栈数字 更新sign

public class Solution {
    public int calculate(String s) {
        s = s.replace(" ", "");
        Stack<Integer> stack = new Stack<Integer>();
        char sign = '+';
        int num = 0 ;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                num = num* 10 + c - '0';
            }
            if (!Character.isDigit(c) || i == s.length() - 1) {
                if (sign == '+') {
                    stack.push(num);
                } else if (sign == '-'){
                    stack.push(-num);
                } else if (sign == '*') {
                    int m = stack.pop();
                    stack.push(m * num);
                } else if (sign == '/') {
                    int m = stack.pop();
                    stack.push(m/num);
                }
                num = 0;
                sign = c;
            }
        }
        int res = 0;
        for (int i : stack) {
            res += i;
        }
        return res;
    }
}