【CodeForces 624C】Graph and String

#include <stdio.h>
#include <algorithm>
#define F(a,b,c) for(int a=b;a<=c;a++)
#define N 505
using namespace std;
int n,m,a[N][N],l[N],u,v,ans,ok;
char s[N];
int main()
{
    scanf("%d%d",&n,&m);
    F(i,1,m){
        scanf("%d%d",&u,&v);
        a[u][v]=a[v][u]=1;
        l[u]++;
        l[v]++;
    }

    F(i,1,n)
        if(l[i]==n-1) s[i]='b';

    int i=1;
    while(s[i]=='b'&&i<=n) i++;

    F(j,1,n)
        if(!a[i][j]) s[j]='c';
        else if(s[j]!='b') s[j]='a';

    s[i]='a';

    F(i,1,n&&ok!=-1)
        F(j,i+1,n)
            if(!a[i][j]&&(s[i]==s[j]||s[i]=='b')
                ||a[i][j]&&(s[i]=='a'&&s[j]=='c'))
                ok=-1;

    if(ok==-1)
        printf("No\n");
    else {
        printf("Yes\n");
        F(j,1,n) printf("%c",s[j]);
    }
    return 0;
}