CF 613C(Necklace-构造法)

#include <iostream>
#include <cmath>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <string>
#include <vector>
#include <map>
#include <functional>
#include <cstdlib>
#include <queue>
#include <stack>
using namespace std;
#define For(i,n) for(int i=1;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 MEM(s) memset(s,0,sizeof(s));
#define Fork(i,k,n) for(int i=k;i<=n;i++)
#define Forp(x) for(int p=pre[x];p;p=next[p])
#define RepD(i,n) for(int i=n;i>=0;i--)
#define pb push_back
const int maxn = 100000 + 10;
int n,a[maxn];
vector<int> v1,v2;
int gcd(int a,int b){if (!b) return a;return gcd(b,a%b);}
void pri(){
For(i,n)
For(j,a[i]) printf("%c",i+'a'-1);
cout<<endl;
}
int main() {
cin>>n;
int tot=0;
For(i,n) {
cin>>a[i];
tot+=a[i];
if (a[i]&1) v1.pb(i);
else if (a[i]) v2.pb(i);
}
int m1=v1.size(),m2=v2.size();
if (m1+m2==1)
{
cout<<tot<<endl;
pri();
return 0;
}
int g=0;
For(i,n) g=gcd(a[i],g);
if (tot&1) {
if (m1>=2) {
cout<<"0"<<endl;
pri();
return 0;
}
cout<<g<<endl;
a[v1[0]]-=g;
For(k,g)
{
For(i,n) For(j,a[i]/g/2) printf("%c",i+'a'-1);
printf("%c",v1[0]+'a'-1);
ForD(i,n) For(j,a[i]/g/2) printf("%c",i+'a'-1);
}
cout<<endl;
} else {
if (m1) {
cout<<"0"<<endl;
pri();
return 0;
}
cout<<g<<endl;
For(k,g)
{

if (k&1) For(i,n) For(j,a[i]/g) printf("%c",i+'a'-1);
else ForD(i,n) For(j,a[i]/g) printf("%c",i+'a'-1);
}
cout<<endl;

}

return 0;
}