poj2591 Set Definition（遞推）

標籤：poj   數學   遞推   

轉載請註明出處：http://blog.csdn.net/u012860063?viewmode=contents

題目連結：http://poj.org/problem?id=2591

Description

Set S is defined as follows: 
(1) 1 is in S; 
(2) If x is in S, then 2x + 1 and 3x + 1 are also in S; 
(3) No other element belongs to S. 

Find the N-th element of set S, if we sort the elements in S by increasing order.

Input

Input will contain several test cases; each contains a single positive integer N (1 <= N <= 10000000), which has been described above.

Output

For each test case, output the corresponding element in S.

Sample Input

100254

Sample Output

4181461

Source

POJ Monthly--2005.08.28,Static

代碼如下：

#include <iostream>using namespace std;int a[10000017];int main(){int i, two = 1, three = 1;a[1] = 1;for(i = 2; i <= 10000000; i++){a[i] = min(a[two]*2+1,a[three]*3+1);if(a[i] == a[two]*2+1)two++;if(a[i] == a[three]*3+1)three++;}int n;while(cin >> n){cout<<a[n]<<endl;}return 0;}