题解 | 牛牛与后缀表达式

import java.util.*;

public class Solution {
    public long legalExp (String str) {
        Deque<Long> stack = new ArrayDeque<>();
        StringBuilder s = new StringBuilder();
        for (int i = 0; i < str.length(); i++) {
            if ('0'<=str.charAt(i)&&str.charAt(i)<='9') {
                s.append(str.charAt(i));
                continue;
            } else if (str.charAt(i) == '#') {
                long n = Long.parseLong(s.toString());
                s.delete(0,s.length());
                stack.push(n);
            } else {
                long a = stack.pop(),b = stack.pop();
                if (str.charAt(i) == '+') {
                    stack.push(a+b);
                } else if (str.charAt(i) == '-') {
                    stack.push(b-a);
                }else{
                    stack.push(a*b);
                }
            }
        }
        return stack.pop();
    }
}