Language: C++

/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <fstream>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>
#include <random>

// container library
#include <array>
#include <bitset>
#include <deque>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

template <typename T>
using PQ = priority_queue<T>;
template <typename T>
using GPQ = priority_queue<T, vector<T>, greater<T>>;

using ll = long long;

#define fst first
#define snd second
#define mp make_pair
#define mt make_tuple

#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)


/* ---------- conversion ---------- */

#define INT(c) static_cast<int>(c)
#define CHAR(n) static_cast<char>(n)
#define LL(n) static_cast<ll>(n)
#define DOUBLE(n) static_cast<double>(n)

/* ---------- container ---------- */

#define ALL(v) (v).begin(), (v).end()
#define SIZE(v) (LL((v).size()))

#define FIND(v, k) (v).find(k) != (v).end()
#define VFIND(v, k) find(ALL(v), k) != (v).end()

#define gsort(b, e) sort(b, e, greater<decltype(*b)>())

/* ----------- debug ---------- */

template <class T>
ostream& operator<<(ostream& os, vector<T> v) {
	os << "[";
	for (auto vv : v)
		os << vv << ",";
	return os << "]";
}

template <class T>
ostream& operator<<(ostream& os, set<T> v) {
	os << "[";
	for (auto vv : v)
		os << vv << ",";
	return os << "]";
}

template <class L, class R>
ostream& operator<<(ostream& os, pair<L, R> p) {
	return os << "(" << p.fst << "," << p.snd << ")";
}

/* ---------- Constants ---------- */

// const ll MOD = 1e9 + 7;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = acos(-1);
// const double EPS = 1e-10;
// mt19937 mert(LL(time(0)));

/* ---------- Short Functions ---------- */

template <typename T>
T sq(T a) {
	return a * a;
}

template <typename T>
T gcd(T a, T b) {
	if (a > b) return gcd(b, a);
	return a == 0 ? b : gcd(b % a, a);
}

template <typename T, typename U>
T mypow(T b, U n) {
	if (n == 0) return 1;
	if (n == 1) return b /* % MOD */;
	if (n % 2 == 0) {
		return mypow(b * b /* % MOD */, n / 2);
	}
	else {
		return mypow(b, n - 1) * b /* % MOD */;
	}
}

ll pcnt(ll b) {
	return __builtin_popcountll(b);
}



// const ll dx[4] = {0, -1, 1, 0};
// const ll dy[4] = {-1, 0, 0, 1};
// const ll dx[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
// const ll dy[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	string S;
	cin >> S;
	char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
	string A;
	A = S;
	int cnt[26] = {};
	REP(j, 26) {
		char c = A[i];
		int n = 0;
		while (0) {
			string B;

			REP(i, A.size() - 1) {
				if (A[i] == c || A[i + 1] == c) {
					B[i] = c;
				}
				else {
					B[i] = A[i];
				}
			}
			set<char> st;
			REP(i, N - 1) st.insert(B[i]);
			if (st.size() == 1) {
				cnt[j] = n;
				break;
			}
			else {
				n++;
				A = B;
			}
		}
	}
	sort(cnt, cnt + 26);
	cout << cnt[0] << endl;
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:141:31: error: 'a' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                               ^
a.cc:141:33: error: 'b' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                 ^
a.cc:141:35: error: 'c' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                   ^
a.cc:141:37: error: 'd' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                     ^
a.cc:141:39: error: 'e' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                       ^
a.cc:141:41: error: 'f' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                         ^
a.cc:141:43: error: 'g' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                           ^
a.cc:141:45: error: 'h' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                             ^
a.cc:141:47: error: 'i' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                               ^
a.cc:141:49: error: 'j' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                 ^
a.cc:141:51: error: 'k' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                   ^
a.cc:141:53: error: 'l' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                     ^
a.cc:141:55: error: 'm' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                       ^
a.cc:141:57: error: 'n' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                         ^
a.cc:141:59: error: 'o' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                           ^
a.cc:141:61: error: 'p' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                             ^
a.cc:141:63: error: 'q' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                               ^
a.cc:141:65: error: 'r' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                 ^
a.cc:141:67: error: 's' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                   ^
a.cc:141:69: error: 't' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                     ^
a.cc:141:71: error: 'u' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                       ^
a.cc:141:73: error: 'v' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                         ^
a.cc:141:75: error: 'w' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                           ^
a.cc:141:77: error: 'x' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                             ^
a.cc:141:79: error: 'y' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                               ^
a.cc:141:81: error: 'z' was not declared in this scope
  141 |         char alphabet[26] = { a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z, };
      |                                                                                 ^
a.cc:160:32: error: 'N' was not declared in this scope
  160 |                         REP(i, N - 1) st.insert(B[i]);
      |                                ^
a.cc:45:44: note: in definition of macro 'FOR'
   45 | #define FOR(i, a, b) for (ll i = (a); i < (b); i++)
      |                                            ^
a.cc:160:25: note: in expansion of macro 'REP'
  160 |                         REP(i, N - 1) st.insert(B[i]);
      |                         ^~~