Codeforces 166E Tetrahedron（dp）

题意：

给你一个正四面体，问你从定点出发走k步后回到定点有几种走法。我反正是不会做，看了CF的题解才明白具体怎么写。我们只需要两个变量，记录当前步数下，走到定点和到其他三个点的种数就好。具体看代码理解。

代码：

////  Created by  CQU_CST_WuErli//  Copyright (c) 2015 CQU_CST_WuErli. All rights reserved.//// #include<bits/stdc++.h>#include <iostream>#include <cstring>#include <cstdio>#include <cstdlib>#include <cctype>#include <cmath>#include <string>#include <vector>#include <list>#include <map>#include <queue>#include <stack>#include <set>#include <algorithm>#include <sstream>#define CLR(x) memset(x,0,sizeof(x))#define OFF(x) memset(x,-1,sizeof(x))#define MEM(x,a) memset((x),(a),sizeof(x))#define For_UVa if (kase!=1) cout << endl#define BUG cout << "I am here" << endl#define lookln(x) cout << #x << "=" << x << endl#define SI(a) scanf("%d",&a)#define SII(a,b) scanf("%d%d",&a,&b)#define SIII(a,b,c) scanf("%d%d%d",&a,&b,&c)#define rep(flag,start,end) for(int flag=start;flag<=end;flag++)#define Rep(flag,start,end) for(int flag=start;flag>=end;flag--)#define Lson l,mid,rt<<1#define Rson mid+1,r,rt<<1|1#define Root 1,n,1#define BigInteger bigntemplate <typename T> T gcd(const T& a,const T& b) {return b==0?a:gcd(b,a%b);}const int MAX_L=2005;// For BigIntegerconst int INF_INT=0x3f3f3f3f;const long long INF_LL=0x7fffffff;const int MOD=1e9+7;const double eps=1e-9;const double pi=acos(-1);typedef long long  ll;using namespace std;int n;int main(){#ifdef LOCAL//  freopen("C:\\Users\\john\\Desktop\\in.txt","r",stdin);//  freopen("C:\\Users\\john\\Desktop\\out.txt","w",stdout);#endif    while (SI(n)==1) {        ll inABC,inD;        inD=1;        inABC=0;        for (int i=1;i<=n;i++) {            ll toD=(inABC*3)%MOD;      // 到定点可以从另外三个点走，所以乘以3。            ll toABC=(inABC*2+inD)%MOD;    // 到三个下面的点之一可以从另外两个点和定点走，所以乘2+定点的。            inD=toD;            inABC=toABC;        }        cout << inD << endl;    }    return 0;}