ICPCCamp 2016 Day1 ftiasch's Contest #4(Around the world-树，构造)

已知一个有n个点(4<=n<=500)连通无向图，边权均为1。点的编号为1,2,...,n，现在你需要给出一条环(v1,v2,…,vn),满足：

• v1,v2,...,vn 是不同的点
• 对2≤i≤n,有dis(vi,vi+1)≤k<n（k是给定的常数）

题解：考虑一棵树，从1号节点出发按dfs序走一遍，下去时走1,3,5..层，上去时走2,4,6,..层，刚好符合题意。

#include<cstdio>#include<cstring>#include<cstdlib>#include<algorithm>#include<functional>#include<iostream>#include<cmath>#include<cctype>#include<ctime>#include<iomanip> using namespace std;#define For(i,n) for(int i=1;i<=n;i++)#define Fork(i,k,n) for(int i=k;i<=n;i++)#define Rep(i,n) for(int i=0;i<n;i++)#define ForD(i,n) for(int i=n;i;i--)#define RepD(i,n) for(int i=n;i>=0;i--)#define Forp(x) for(int p=pre[x];p;p=next[p])#define Forpiter(x) for(int &p=iter[x];p;p=next[p])  #define Lson (x<<1)#define Rson ((x<<1)+1)#define MEM(a) memset(a,0,sizeof(a));#define MEMI(a) memset(a,127,sizeof(a));#define MEMi(a) memset(a,128,sizeof(a));#define INF (2139062143)#define F (100000007)#define pb push_back#define mp make_pair#define fi first#define se second#define vi vector<int> typedef long long ll;ll mul(ll a,ll b){return (a*b)%F;}ll add(ll a,ll b){return (a+b)%F;}ll sub(ll a,ll b){return (a-b+llabs(a-b)/F*F+F)%F;}void upd(ll &a,ll b){a=(a%F+b%F)%F;}int read(){    int x=0,f=1; char ch=getchar();    while(!isdigit(ch)) {if (ch=='-') f=-1; ch=getchar();}    while(isdigit(ch)) { x=x*10+ch-'0'; ch=getchar();}    return x*f;} #define MAXN (500+10)int n,k;char s[MAXN];int f[MAXN][MAXN]={0};bool vis[MAXN]={0};void dfs(int x,int l){    vis[x]=1;    if (!l) {        if (x>1) cout<<" ";        cout<<x;    }    For(i,n) if (!vis[i]&&f[x][i]) {        dfs(i,l^1);    }    if (l) cout<<' '<<x;}int main(){//  freopen("D.in","r",stdin);//  freopen(".out","w",stdout);    n=read(),k=read();    For(i,n) {        cin>>(s+1);        For(j,n) if (s[j]=='1') f[i][j]=1;    }    dfs(1,0);    cout<<endl;    return 0;}