HDU5424

【题意】有一张n个点n条边的无向图，这张图是否存在一条哈密顿路径。

【分析】关键是只有n条边，不然求哈密顿路径是个NP问题；这样去掉重边和自环后如果图联通那么如果度数为1得点必然不能在中间经过，所以每次选择度数最小的点dfs访问一遍，如果能访问所有点则存在哈密顿路径，否则不存在。复杂度O(n)

【AC CODE】0ms

#include <cstdio>
#include <cstring>
#include <cctype>
#include <cmath>
#include <set>
#include <bitset>
//#include <unordered_set>
#include <queue>
#include <stack>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
typedef long long LL;
#define rep(i,a,n) for(int i = a; i < n; i++)
#define repe(i,a,n) for(int i = a; i <= n; i++)
#define per(i,n,a) for(int i = n; i >= a; i--)
#define clc(a,b) memset(a,b,sizeof(a))
const int INF = 0x3f3f3f3f, MAXIN = 10000, MAXOUT = 10000, MAXN = 1000+10, MAXM = 1000*2+10;
char buf[MAXIN], *ps = buf, *pe = buf+1;
inline void rnext(){
	if(++ps == pe) pe = (ps = buf)+fread(buf,sizeof(char),sizeof(buf)/sizeof(char),stdin);
}
template <class T>
inline bool in(T &ans)
{
	ans = 0;
	T f = 1;
	if(ps == pe) return false;
	do{ rnext(); if('-' == *ps) f = -1;} while(!isdigit(*ps) && ps != pe);
	if(ps == pe) return false;
	do{ ans = (ans<<1)+(ans<<3)+*ps-48;rnext();}while(isdigit(*ps) && ps != pe);
	ans *= f;
	return true;
}
char bufout[MAXOUT], outtmp[50],*pout = bufout, *pend = bufout+MAXOUT;
inline void write(){ fwrite(bufout,sizeof(char),pout-bufout,stdout);pout = bufout;}
inline void out_char(char c){ *(pout++) = c;if(pout == pend) write();}
inline void out_str(char *s)
{
	while(*s){ *(pout++) = *(s++); if(pout == pend) write(); }
}
template <class T>
inline void out_int(T x) {
	if(!x){ out_char('0');return;}
	if(x < 0) x = -x,out_char('-');
	int len = 0;
	while(x){ outtmp[len++] = x%10+48; x /= 10;}
	outtmp[len] = 0;
	for(int i = 0, j = len-1; i < j; i++,j--) swap(outtmp[i],outtmp[j]);
	out_str(outtmp);
}
int head[MAXN],tol,nxt[MAXM],to[MAXM];

inline void add_edge(int u, int v)
{
	nxt[tol] = head[u];to[tol] = v;
	head[u] = tol++;
}
int du[MAXN],cnt,n;
bool vis[MAXN];
void dfs(int u)
{
	vis[u] = true;
	cnt++;
	int v = -1;
	for(int i = head[u]; ~i; i = nxt[i])
	{
		if(vis[to[i]]) continue;
		if(-1 == v || du[to[i]] < du[v]) v = to[i];
	}
	if(~v) dfs(v);
}
int main()
{
#ifdef SHY
	freopen("d:\\1.txt", "r", stdin);
#endif
	while(in(n))
	{
		clc(head,-1);tol = 0;
		clc(du,0);
		rep(i,0,n)
		{
			int u,v;
			in(u);in(v);
			du[u]++,du[v]++;
			add_edge(u,v);
			add_edge(v,u);
		}
		int u = 1;
		repe(i,2,n) if(du[i] < du[u]) u = i;
		cnt = 0;
		clc(vis,0);
		dfs(u);
		if(cnt == n) out_str("YES\n");
		else out_str("NO\n");
	}
	write();
	return 0;
}