UVA Find the Permutations 11077 (DP&置换群)
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Find the Permutations
Sorting is one of the most used operations in real life, where Computer Science comes into act. It iswell-known that the lower bound of swap based sorting is nlog(n). It means that the best possiblesorting algorithm will take at least O(nlog(n)) swaps to sort a set of n integers. However, to sort aparticular array of n integers, you can always find a swapping sequence of at most (n − 1) swaps, onceyou know the position of each element in the sorted sequence.For example consider four elements <1 2 3 4>. There are 24 possible permutations and for allelements you know the position in sorted sequence.If the permutation is <2 1 4 3>, it will take minimum 2 swaps to make it sorted. If the sequenceis <2 3 4 1>, at least 3 swaps are required. The sequence <4 2 3 1> requires only 1 and the sequence<1 2 3 4> requires none. In this way, we can find the permutations of N distinct integers which willtake at least K swaps to be sorted.
Input
Each input consists of two positive integers N (1 ≤ N ≤ 21) and K (0 ≤ K < N) in a single line.Input is terminated by two zeros. There can be at most 250 test cases.
Output
For each of the input, print in a line the number of permutations which will take at least K swaps.
Sample Input
3 1
3 0
3 2
0 0
Sample Output
3
1
2
//题意:
给出两个数n,k,对1-n的所有序列进行两两交换,统计有多少个序列至少需要进行k次操作才能变成(1,2,......n)。
//思路:
看到这个题首先就会先到置换,单元素循环是不需要交换,两个元素循环要交换一次,.......n个元素循环要交换n-1次。
这样的话,如果序列的循环节为x,总共交换的次数为n-x次。所以可以退出下面:
用dp[i][j]表示至少要进行j次操作才能将1-i,这i个数排列为(1,2,......i)这样的序列的个数。
所以就会得到方程:dp[i][j]=dp[i-1][j-1]+f[i-1][j]*(i-1)。
因为元素i要么自己形成一个循环,要么加入到前面任意一个为。
#include<stdio.h>#include<string.h>#include<algorithm>#define INF 0x3f3f3f3f#define ll unsigned long long#define N 30using namespace std;int gcd(int a,int b){return b==0?a:gcd(b,a%b);}ll dp[N][N];int init(){int i,j;memset(dp,0,sizeof(dp));dp[1][0]=1;for(i=2;i<=21;i++){for(j=0;j<i;j++){dp[i][j]=dp[i-1][j];if(j>0)dp[i][j]+=dp[i-1][j-1]*(i-1);}}}int main(){init();int n,k;while(scanf("%d%d",&n,&k),n|k){printf("%llu\n",dp[n][k]);}return 0;}
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