Bubble (opentrains)

规律题
要求生成一个1-n的序列，冒泡排序后每个数字都只交换两次
通过一个dp发现只要数列能够分成3*x+4*y的形式就可以生成
3位的[1,2,3]变成[3,2,1]
4位的[1,2,3,4]变成[3,4,1,2]
按照这个规律就可以了

#include <bits/stdc++.h>using namespace std;const int maxn = 110;int f[maxn], n, a[maxn], l, r, x, y;int main() {    freopen("input.txt","r",stdin);    freopen("output.txt","w",stdout);    scanf("%d", &n);    f[3] = 1;    f[4] = 1;    for (int i = 5; i <= n; i++) f[i] = f[i-3]+f[i-4];    printf("%d\n", f[n]);    if (f[n] != 0) {        x = 0;        while (1) {            if ((n-3*x)%4 == 0) {                y = (n-3*x)/4;                break;            }            x++;        }        l = 1;        for (int i = 1; i <= x; i++) {            r = l+2;            a[l] = l+2;            a[l+1] = l+1;            a[l+2] = l;            l = l+3;        }        for (int i = 1; i <= y; i++) {            r = l+3;            a[l] = l+2;            a[l+1] = l+3;            a[l+2] = l;            a[l+3] = l+1;            l = l+4;        }        for (int i = 1; i < n; i++) printf("%d ", a[i]);        printf("%d\n", a[n]);    }}