codeforces 623A. Graph and String 构造

#include <iostream>
#include <vector>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <map>
#include <set>
#include <string>
#include <queue>
#include <stack>
#include <bitset>
using namespace std;
#define pb(x) push_back(x)
#define ll long long
#define mk(x, y) make_pair(x, y)
#define lson l, m, rt<<1
#define mem(a) memset(a, 0, sizeof(a))
#define rson m+1, r, rt<<1|1
#define mem1(a) memset(a, -1, sizeof(a))
#define mem2(a) memset(a, 0x3f, sizeof(a))
#define rep(i, n, a) for(int i = a; i<n; i++)
#define fi first
#define se second
typedef pair<int, int> pll;
const double PI = acos(-1.0);
const double eps = 1e-8;
const int mod = 1e9+7;
const int inf = 1061109567;
const int dir[][2] = { {-1, 0}, {1, 0}, {0, -1}, {0, 1} };
int a[260000], b[260000], de[505], c[505];
int mp[505][505];
int main()
{
    int n, m, pos = 0;
    cin>>n>>m;
    for(int i = 0; i<m; i++) {
        scanf("%d%d", &a[i], &b[i]);
        de[a[i]]++, de[b[i]]++;
        mp[a[i]][b[i]] = mp[b[i]][a[i]] = 1;
    }
    for(int i = 1; i<=n; i++) {
        if(de[i] == n-1) {
            c[i] = 2;
        }
    }
    for(int i = 1; i<=n; i++) {
        if(!c[i]) {
            pos = i;
            c[i] = 1;
            break;
        }
    }
    if(!pos) {
        puts("Yes");
        for(int i = 0; i<n; i++)
            cout<<‘b‘;
        return 0;
    }
    for(int i = 1; i<=n; i++) {
        if(i == pos)
            continue;
        if(!mp[i][pos])
            c[i] = 3;
        else if(!c[i])
            c[i] = 1;
    }
    int flag = 0;
    for(int i = 1; i<=n; i++) {
        for(int j = i+1; j<=n; j++) {
            if(abs(c[i]-c[j])>1&&mp[i][j]) {
                flag = 1;
            }
            if(abs(c[i]-c[j])<=1 && !mp[i][j]) {
                flag = 1;
            }
        }
    }
    if(flag) {
        puts("No");
        return 0;
    } else {
        puts("Yes");
        for(int i = 1; i<=n; i++) {
            cout<<char(‘a‘+c[i]-1);
        }
        cout<<endl;
    }
    return 0;
}