1 #include <iostream> 2 #include <queue> 3 #include <stack> 4 #include <cstdio> 5 #include <vector> 6 #include <map> 7 #include <set> 8 #include <bitset> 9 #include <algorithm> 10 #include <cmath> 11 #include <cstring> 12 #include <cstdlib> 13 #include <string> 14 #include <sstream> 15 #include <time.h> 16 #define x first 17 #define y second 18 #define pb push_back 19 #define mp make_pair 20 #define lson l,m,rt*2 21 #define rson m+1,r,rt*2+1 22 #define mt(A,B) memset(A,B,sizeof(A)) 23 #define lowbit(x) (x&(-x)) 24 using namespace std; 25 typedef long long LL; 26 //const double PI = acos(-1); 27 const int N=2e5+10; 28 //const int M=1e6+10; 29 const LL mod=1e9+9; 30 const int inf = 0x3f3f3f3f; 31 const LL INF=0x3f3f3f3f3f3f3f3fLL; 32 const double esp=1e-10; 33 const int MAXN=500010; 34 int vis[N],top=0,head[N],ans[N],pre[N],vik[2*N],cnt=0; 35 struct node 36 { 37 int u,v,next; 38 }E[N*2]; 39 void add(int u,int v) 40 { 41 E[top].u=u; 42 E[top].v=v; 43 E[top].next=head[u]; 44 head[u]=top++; 45 } 46 int found(int x) 47 { 48 if(x==pre[x])return x; 49 else return pre[x]=found(pre[x]); 50 } 51 void unite(int x,int y) 52 { 53 int fx=found(x); 54 int fy=found(y); 55 if(fx!=fy)pre[fx]=fy; 56 } 57 void dfs(int u) 58 { 59 while(head[u]!=-1) 60 { 61 if(!vik[head[u]]) 62 { 63 vik[head[u]]=1; 64 vik[head[u]^1]=1; 65 int i=head[u]; 66 dfs(E[i].v); 67 ans[cnt++]=i; 68 } 69 else head[u]=E[head[u]].next; 70 } 71 } 72 int main() 73 { 74 #ifdef Local 75 freopen("data.txt","r",stdin); 76 #endif 77 //ios::sync_with_stdio(false); 78 //cin.tie(0); 79 int n,m,u,v,flag=0,num=0; 80 scanf("%d%d",&n,&m); 81 mt(vis,0); 82 mt(vik,0); 83 mt(head,-1); 84 for(int i=1;i<=n;i++)pre[i]=i; 85 for(int i=1;i<=m;i++) 86 { 87 scanf("%d%d",&u,&v); 88 add(u,v); 89 add(v,u); 90 unite(u,v); 91 vis[u]++;vis[v]++; 92 } 93 for(int i=1;i<=n;i++) 94 { 95 if(vis[i]%2)flag=1; 96 } 97 for(int i=1;i<=n;i++) 98 { 99 if(pre[i]==i)num++; 100 } 101 if(num>1)flag=1; 102 if(flag) 103 { 104 puts("NO");return 0; 105 } 106 else 107 { 108 dfs(1); 109 puts("YES"); 110 sort(ans,ans+m); 111 for(int i=0;i<m;i++) 112 { 113 printf("%d %d ",E[ans[i]].u,E[ans[i]].v); 114 } 115 } 116 return 0; 117 #ifdef Local 118 cerr << "time: " << (LL) clock() * 1000 / CLOCKS_PER_SEC << " ms" << endl; 119 #endif 120 }