Poj 3126 Prime Path
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1.Link:
http://poj.org/problem?id=3126
2.Content:
Prime PathTime Limit: 1000MS Memory Limit: 65536KTotal Submissions: 11757 Accepted: 6675Description
The ministers of the cabinet were quite upset by the message from the Chief of Security stating that they would all have to change the four-digit room numbers on their offices.
— It is a matter of security to change such things every now and then, to keep the enemy in the dark.
— But look, I have chosen my number 1033 for good reasons. I am the Prime minister, you know!
— I know, so therefore your new number 8179 is also a prime. You will just have to paste four new digits over the four old ones on your office door.
— No, it’s not that simple. Suppose that I change the first digit to an 8, then the number will read 8033 which is not a prime!
— I see, being the prime minister you cannot stand having a non-prime number on your door even for a few seconds.
— Correct! So I must invent a scheme for going from 1033 to 8179 by a path of prime numbers where only one digit is changed from one prime to the next prime.
Now, the minister of finance, who had been eavesdropping, intervened.
— No unnecessary expenditure, please! I happen to know that the price of a digit is one pound.
— Hmm, in that case I need a computer program to minimize the cost. You don't know some very cheap software gurus, do you?
— In fact, I do. You see, there is this programming contest going on... Help the prime minister to find the cheapest prime path between any two given four-digit primes! The first digit must be nonzero, of course. Here is a solution in the case above.1033The cost of this solution is 6 pounds. Note that the digit 1 which got pasted over in step 2 can not be reused in the last step – a new 1 must be purchased.
1733
3733
3739
3779
8779
8179Input
One line with a positive number: the number of test cases (at most 100). Then for each test case, one line with two numbers separated by a blank. Both numbers are four-digit primes (without leading zeros).Output
One line for each case, either with a number stating the minimal cost or containing the word Impossible.Sample Input
31033 81791373 80171033 1033Sample Output
670Source
Northwestern Europe 2006
3.Method:
4.Code:
1 #include<iostream> 2 #include<queue> 3 using namespace std; 4 #define MAX 10002 5 //#define MAX 102 6 int a[MAX]; 7 int b[MAX]; 8 int main() 9 {10 int n,m;11 int i,j;12 int x,y;13 int k;14 queue<int> q;15 a[2]=0; 16 for(i=4;i<MAX;i+=2) a[i]=1; 17 for(i=3;i<=MAX/2;i++)18 {19 if(a[i]==0)20 {21 for(j=i;j<MAX/i;j+=2)22 {23 a[i*j]=1; 24 }25 }26 }27 //验证素数表的正确性28 /*for(i=2;i<MAX;i++)29 {30 if(a[i]==1) printf("%d是素数\n",i);31 else printf("%d不是素数\n",i);32 }*/ 33 cin>>n;34 for(int ii=0;ii<n;ii++)35 {36 cin>>x>>y;37 for(j=0;j<MAX;j++) b[j]=-1;38 while(!q.empty()) q.pop();39 q.push(x);40 b[x]=0;41 while(!q.empty())42 {43 m=q.front();44 if(m==y) break;45 else46 {47 for(i=0;i<4;i++)48 {49 k=1;50 for(j=0;j<i;j++) k=k*10;51 for(j=-9;j<=9;j++)52 {53 if(((m/(k*10))==((m+j*k)/(k*10)))&&(a[m+j*k]==0)&&(b[m+j*k]==-1)&&(m+j*k)>=1000)54 {55 //printf("perfect");56 b[m+j*k]=b[m]+1;57 q.push(m+j*k);58 }59 }60 }61 }62 q.pop();63 }64 cout<<b[y]<<endl; 65 }66 //system("pause");67 return 1;68 }
5.Reference:
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