Codeforces AIM TECT Round#1 B DP
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题意:给一个序列,有两种操作,1.删除连续的一段(不能删除整个序列),2.把其中一些数+1,或者-1。只能删除一段,删除和改变都有一个权值。要求:删除或改变之后(可以什么都不做)这个序列的gcd值大于1。而且权值最小。
思路:在CF上找了代码参考,dp方程看不懂,然后就自己写了dp方程了。首先,由不能删除整个序列,所以这个序列的gcd值一定是v[1],v[1]-1,v[1]+1,v[n],v[n]-1,v[n]+1的一个质因子。我的dp思路是看以什么结尾。以删除这个数作为结尾,改变这个数作为结尾,还是不变作为结尾。dp方程的意义代码里有写。
http://codeforces.com/contest/623/problem/B
/********************************************* Problem : AIM TECT Round#1 B Author : NMfloat InkTime (c) NM . All Rights Reserved .********************************************/#include <map>#include <set>#include <queue>#include <stack>#include <cmath>#include <ctime>#include <cstdio>#include <vector>#include <cstring>#include <cstdlib>#include <iostream>#include <algorithm>#define rep(i,a,b) for(LL i = (a) ; i <= (b) ; i ++) //遍历#define rrep(i,a,b) for(LL i = (b) ; i >= (a) ; i --) //反向遍历#define repS(it,p) for(auto it = p.begin() ; it != p.end() ; it ++) //遍历一个STL容器#define repE(p,u) for(Edge * p = G[u].first ; p ; p = p -> next) //遍历u所连接的点#define cls(a,x) memset(a,x,sizeof(a))#define eps 1e-8using namespace std;const int MOD = 1e9+7;// const LL INF = 0x3f3f3f3f;const int MAXN = 1e6+5;const int MAXE = 2e5+5;typedef long long LL;typedef unsigned long long ULL;LL T,n,m;LL fx[] = {0,1,-1,0,0};LL fy[] = {0,0,0,-1,1};LL remove_cost , change_cost;LL v[MAXN];LL cost[MAXN];LL dp[MAXN][3][2];LL INF = (LL)1e18;void factor(LL num , vector<LL> & p) { //获取一个数的质因子,并存进一个数组里。 for(LL i = 2 ; i * i <= num ; i ++) { if(num % i == 0) { p.push_back(i); while(num % i == 0) num /= i; } } if(num != 1) p.push_back(num);}void input() { rep(i,1,n) scanf("%I64d",&v[i]);}void solve() { vector<LL>p; factor(v[1],p); factor(v[1]-1,p); factor(v[1]+1,p); factor(v[n],p); factor(v[n]-1,p); factor(v[n]+1,p); sort(p.begin(), p.end());//排序 p.erase(unique(p.begin(), p.end()),p.end()); //消除重复 LL ret = INF; repS(it,p) { LL prime = *it; rep(i,1,n) { if(v[i] % prime == 0) cost[i] = 0; else if((v[i]-1) % prime == 0 || (v[i] + 1) % prime == 0) cost[i] = 1; else cost[i] = 2; } if(cost[1] == 2 && cost[n] == 2) continue; //dp[i][0][0] 代表以cost[i] == 0为结尾 之前没有删除过元素的最小代价 //dp[i][0][1] 代表以cost[i] == 0为结尾 之前删除过一段元素的最小代价 //dp[i][1][0] 代表以cost[i] == 1为结尾 之前没有删除过元素的最小代价 //dp[i][1][1] 代表以cost[i] == 1为结尾 之前删除过一段元素的最小代价 //dp[i][2][0] 代表以cost[i] == 2为结尾 之前删除或没有删除过元素的最小代价 //dp[i][2][1] 无意义 rep(k,1,n) rep(i,0,2) rep(j,0,1) dp[k][i][j] = INF; dp[0][0][0] = 0; rep(i,1,n) { if(cost[i] == 0) { dp[i][0][0] = min(dp[i-1][0][0],dp[i-1][1][0]); dp[i][0][1] = min(dp[i-1][0][1],min(dp[i-1][1][1],dp[i-1][2][0])); dp[i][2][0] = min(dp[i-1][0][0],min(dp[i-1][1][0],dp[i-1][2][0])) + remove_cost; } else if(cost[i] == 1) { dp[i][1][0] = min(dp[i-1][0][0],dp[i-1][1][0]) + change_cost; dp[i][1][1] = min(dp[i-1][0][1],min(dp[i-1][1][1],dp[i-1][2][0])) + change_cost; dp[i][2][0] = min(dp[i-1][0][0],min(dp[i-1][1][0],dp[i-1][2][0])) + remove_cost; } else if(cost[i] == 2) { dp[i][2][0] = min(dp[i-1][0][0],min(dp[i-1][1][0],dp[i-1][2][0])) + remove_cost; } } rep(i,0,2) rep(j,0,1) ret = min(ret,dp[n][i][j]); } printf("%I64d\n",ret);}int main(void) { //freopen("a.in","r",stdin); while(~scanf("%I64d %I64d %I64d",&n,&remove_cost,&change_cost)) { input(); solve(); } return 0;}
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