栈系列一＞基本计算器II

class Solution {
    public int calculate(String _s) {
        //isDigit方法: 判断字符是否是数字
        
        char[] s = _s.toCharArray();
        int n = s.length;
        int i = 0;
        char op = '+';

        Deque<Integer> st = new ArrayDeque<>();

        while(i < n){
            if(s[i] == ' ') i++;
            else if(Character.isDigit(s[i])) {
                int tmp = 0;
                while(i < n && Character.isDigit(s[i])){
                    tmp = tmp * 10 + (s[i] - '0');
                    i++;
                }

                if(op == '+') st.push(tmp);
                else if(op == '-') st.push(-tmp);
                else if(op == '*') st.push(st.pop() * tmp);
                else if(op == '/') st.push(st.pop() / tmp);
            }else {
                op = s[i];
                i++;
            }
        }
        int ret = 0;
        while(!st.isEmpty()){
            ret += st.pop();
        }

        return ret;
    }
}