暴力重排
（注意重拍第一个字母
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <cstring>
#include <climits>
#include <complex>
#include <fstream>
#include <cassert>
#include <cstdio>
#include <bitset>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <ctime>
#include <set>
#include <map>
#include <cmath>
#define CLR(x,y) memset(x,y,sizeof(x))
#define mp(x,y) make_pair(x,y)
#define eps 1e-9
#define INF 0x3f3f3f3f
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef complex<ld> point;
typedef pair<int, int> pii;
typedef pair<pii, int> piii;
template<class T>
inline bool read(T &n)
{
T x = 0, tmp = 1; char c = getchar();
while((c < '0' || c > '9') && c != '-' && c != EOF) c = getchar();
if(c == EOF) return false;
if(c == '-') c = getchar(), tmp = -1;
while(c >= '0' && c <= '9') x *= 10, x += (c - '0'),c = getchar();
n = x*tmp;
return true;
}
template <class T>
inline void write(T n)
{
if(n < 0)
{
putchar('-');
n = -n;
}
int len = 0,data[20];
while(n)
{
data[len++] = n%10;
n /= 10;
}
if(!len) data[len++] = 0;
while(len--) putchar(data[len]+48);
}
//-----------------------------------
string a;
int n,p;
bool dfs(int i)
{
if(i>=a.length())
return 1;
if(i==n-1)
a.at(n-1)++;
for(;a.at(i)<=p+'a'-1;a.at(i)++)
{
if(a.at(i)!=a.at(i-1) && !(i>1&&a.at(i-2)==a.at(i)) && dfs(i+1))
return 1;
}
a.at(i)='a';
if(i==n-1)
a.at(i)--;
return 0;
}
int main()
{
read(n),read(p);
cin>>a;
if(n==1)
{
if(a.at(0)=='a'+p-1)
puts("NO");
else
cout<<(char)(a.at(0)+1)<<endl;
return 0;
}
for(;a.at(0)<='a'+p-1;a.at(0)++)
if(dfs(1))
goto OK;
goto No;
OK:
cout<<a<<endl;
return 0;
No:
cout<<"NO"<<endl;
return 0;
}