【codeforces】 464A No to Palindromes!

额。。。一个暴力题。。。还想了好一会儿。。

#include <iostream>  
#include <queue>  
#include <stack>  
#include <map>  
#include <set>  
#include <bitset>  
#include <cstdio>  
#include <algorithm>  
#include <cstring>  
#include <climits>  
#include <cstdlib>
#include <cmath>
#include <time.h>
#define maxn 400005
#define maxm 400005
#define eps 1e-10
#define mod 1000000007
#define INF 999999999
#define lowbit(x) (x&(-x))
#define mp mark_pair
#define ls o<<1
#define rs o<<1 | 1
#define lson o<<1, L, mid  
#define rson o<<1 | 1, mid+1, R  
//typedef vector<int>::iterator IT;
typedef long long LL;
//typedef int LL;
using namespace std;
LL qpow(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base;base=base*base;b/=2;}return res;}
LL powmod(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base%mod;base=base*base%mod;b/=2;}return res;}
void scanf(int &__x){__x=0;char __ch=getchar();while(__ch==' '||__ch=='\n')__ch=getchar();while(__ch>='0'&&__ch<='9')__x=__x*10+__ch-'0',__ch = getchar();}
LL gcd(LL _a, LL _b){if(!_b) return _a;else return gcd(_b, _a%_b);}
// head

char s[maxn];
int n, p;
int dfs(int now)
{
	if(now == 1 && s[now] == 'a' + p - 1) return 0;
	if(s[now] == 'a' + p - 1) return dfs(now-1);
	for(char i = s[now]+1; i <= 'a' + p - 1; i++)
		if((!(now-1) || s[now-1] != i) && (!(now-2) || s[now-2] != i)) return now;
	return dfs(now-1);
}
int main(void)
{
	scanf("%d%d", &n, &p);
	scanf("%s", s+1);
	int tmp = dfs(n);
	if(!tmp) {
		printf("NO\n");
		return 0;
	}
	char j;
	for(j = s[tmp]+1; j <= 'a' + p - 1; j++)
		if((!(tmp-1) || s[tmp-1] != j) && (!(tmp-2) || s[tmp-2] != j)) break;
	s[tmp] = j;
	for(int i = tmp+1; i <= n; i++) {
		for(j = 'a'; j <= 'a' + p - 1; j++)
			if((!(i-1) || s[i-1] != j) && (!(i-2) || s[i-2] != j)) break;
		s[i] = j;
	}
	puts(s+1);
	return 0;
}