牛客多校第九场 Groundhog and 2-Power Representation（大整数，java）

思路：
直接java模拟递归写。
不过python貌似可以用语法直接写。

import java.util.*;
import java.math.*;
import java.io.*;
import java.lang.*;

public class Main {
    public static BigInteger qpow(BigInteger n) {
        BigInteger res = BigInteger.ONE;
        BigInteger zero = BigInteger.ZERO;
        BigInteger one = BigInteger.ONE;
        BigInteger two = one.add(one);
        BigInteger x = two;
        while(n != zero) {
            if(n.mod(two).equals(one)) {
                res = res.multiply(x);
            }
            n = n.divide(two);
            x = x.multiply(x);
        }
        return res;
    }

    public static BigInteger cal(int l,int r,String s) {
        BigInteger zero = BigInteger.ZERO;
        BigInteger one = BigInteger.ONE;
        BigInteger two = one.add(one);

        if(s.charAt(l) == '(' && s.charAt(r) == ')') {
            l++;
            r--;
        }
        if(l == r) {
            if(s.charAt(l) == '0') return zero;
            else if(s.charAt(l) == '2') return two;
        }

        int pre = l;
        BigInteger res = BigInteger.ZERO;
        int cnt = 0;
        for(int i = l;i <= r;i++) {
            if(s.charAt(i) == '(') cnt--;
            else if(s.charAt(i) == ')') cnt++;
            else if(s.charAt(i) == '+' && cnt == 0) {
                BigInteger num = cal(pre,i - 1,s);
//                System.out.println(num + " " + pre + " " + (i - 1));
                res = res.add(num);
                pre = i + 1;
            }
        }
        if(pre < r) pre++;
        BigInteger num = cal(pre,r,s);
        if(r - pre + 1 != 1) num = qpow(num);
        res = res.add(num);

        return res;
    }

    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        String s;
        s = cin.next();
        int len = s.length();
        BigInteger ans = cal(0,len - 1,s);
        System.out.println(ans);
    }
}



//2(2+2(0))