A：差点开场懵逼。只要有相邻两位不同就可以作为答案。
#include<iostream> #include<cstdio> #include<cmath> #include<cstdlib> #include<cstring> #include<algorithm> using namespace std; int read() { int x=0,f=1;char c=getchar(); while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();} while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar(); return x*f; } #define N 1010 int n,cnt[N][26]; char s[N]; int main() { n=read(); scanf("%s",s+1); for (int i=1;i<n;i++) if (s[i]!=s[i+1]) {cout<<"YES\n"<<s[i]<<s[i+1];return 0;} cout<<"NO"; return 0; }