UVA - 10862 Connect the Cable Wires

题意：看了大神的推理：点击打开链接

然后得到公式f(n)=f(n-1)*3-f(n-2) (n>2)，之后便是大数的计算

#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;
const int MAXN = 2005;

int len[MAXN],n;
int f[MAXN][MAXN];

void mul(int a,int b){
    int i,t = 0;
    len[a] = len[b];
    for (i = 0; i < len[a]; i++){
        f[a][i] = f[b][i] * 3 + t;
        t = f[a][i]/10;
        f[a][i] %= 10;
    }
    if (t){
        len[a]++;
        f[a][i] = t;
    }
}

void sub(int a,int b){
    int t = 0;
    for (int i = 0; i < len[a]; i++){
        f[a][i] = f[a][i] - f[b][i] + t;
        if (f[a][i] < 0){
            f[a][i] += 10;
            t = -1;
        }
        else t = 0;
    }
    while (f[a][len[a]-1] == 0)
        len[a]--;
}

int main(){
    memset(f,0,sizeof(f));
    f[1][0]=1,f[2][0]=3;
    len[1] = len[2] = 1;
    for (int i = 3; i <= 2000; i++){
        mul(i,i-1);
        sub(i,i-2);
    }
    while (scanf("%d",&n) != EOF && n){
        for (int i = len[n]-1; i >= 0; i--)
            printf("%d",f[n][i]);
        printf("\n");
    }
    return 0;
}