51nod 1832 先序遍历与后序遍历（dfs+高精度）

 http://www.51nod.com/onlineJudge/questionCode.html#!problemId=1832

题意：

思路：

官方题解如下：

可以看一下这篇文章：https://wenku.baidu.com/view/a2a45aa0284ac850ad024261.html

#include<iostream>
#include<cstdio>
#include<cstring>
#include<queue>
using namespace std;
const int maxn = 10000+5;
const int inf = 0x3f3f3f3f;

int n;
int a[maxn],b[maxn];

struct BigInt
{
    const static int mod = 10000;
    const static int DLEN = 4;
    int a[600],len;
    BigInt()
    {
        memset(a,0,sizeof(a));
        len = 1;
    }
    BigInt(int v)
    {
        memset(a,0,sizeof(a));
        len = 0;
        do
        {
            a[len++] = v%mod;
            v /= mod;
        }while(v);
    }
    BigInt(const char s[])
    {
        memset(a,0,sizeof(a));
        int L = strlen(s);
        len = L/DLEN;
        if(L%DLEN)len++;
        int index = 0;
        for(int i = L-1;i >= 0;i -= DLEN)
        {
            int t = 0;
            int k = i - DLEN + 1;
            if(k < 0)k = 0;
            for(int j = k;j <= i;j++)
                t = t*10 + s[j] - '0';
            a[index++] = t;
        }
    }
    BigInt operator +(const BigInt &b)const
    {
        BigInt res;
        res.len = max(len,b.len);
        for(int i = 0;i <= res.len;i++)
            res.a[i] = 0;
        for(int i = 0;i < res.len;i++)
        {
            res.a[i] += ((i < len)?a[i]:0)+((i < b.len)?b.a[i]:0);
            res.a[i+1] += res.a[i]/mod;
            res.a[i] %= mod;
        }
        if(res.a[res.len] > 0)res.len++;
        return res;
    }
    BigInt operator *(const BigInt &b)const
    {
        BigInt res;
        for(int i = 0; i < len;i++)
        {
            int up = 0;
            for(int j = 0;j < b.len;j++)
            {
                int temp = a[i]*b.a[j] + res.a[i+j] + up;
                res.a[i+j] = temp%mod;
                up = temp/mod;
            }
            if(up != 0)
                res.a[i + b.len] = up;
        }
        res.len = len + b.len;
        while(res.a[res.len - 1] == 0 &&res.len > 1)res.len--;
        return res;
    }
    void output()
    {
        printf("%d",a[len-1]);
        for(int i = len-2;i >=0 ;i--)
            printf("%04d",a[i]);
        printf("
");
    }
};
BigInt ans;

void dfs(int al, int ar, int bl, int br)
{
    if(ar-al<=0)  return;
    al++;
    br--;
    int cnt = 0;
    int index = bl;
    while(a[al]!=b[index])  index++;
    int newar = al+index-bl+1;
    int newbl = index+1;
    cnt++;
    dfs(al,newar-1,bl,index);
    if(ar-al!=index-bl)
    {
        cnt++;
        dfs(newar,ar,newbl,br);
    }
    if(cnt==1)  ans=ans*2;
}

int main()
{
    //freopen("in.txt","r",stdin);
    ans = 1;
    scanf("%d",&n);
    for(int i=0;i<n;i++)  scanf("%d",&a[i]);
    for(int i=0;i<n;i++)  scanf("%d",&b[i]);
    dfs(0,n-1,0,n-1);
    ans.output();
    return 0;
}