Language: C++

//#include <bits/stdc++.h>
#include <iostream>
#include <algorithm>
#pragma warning (disable:4996)
#ifndef constexpr
#define constexpr const
#endif // !constexpr

using namespace std;
using ll = long long;
using LL = ll;
using ull = unsigned long long;
using uLL = ull;
using pii = pair<int, int>;
#define clean_0(A) memset((A),0,sizeof(A))
#define clean_1(A) memset((A),-1,sizeof(A))
constexpr int INF = 0x7F7F7F7F;
#define clean_INF(A) memset((A),INF,sizeof(A))
#define For(i,a,b) for(int (i)=(a);(i)<=b;++(i))

constexpr double eps = 1e-9;

constexpr ll mod1 = 197;
constexpr ll mod2 = 19260817;
constexpr ll mod3 = ll(1e8) + 7;
constexpr ll mod4 = ll(1e9) + 7;;
constexpr ll mod5 = 998244353;
constexpr ll mod = mod4;
constexpr int maxn = int(2e6) + 1;
constexpr int MAXN = maxn;
//#define Enable_IO
//#define Enable_MATH

#ifdef Enable_IO
namespace io
{
#define in(a) (a=read())
#define out(a) write(a)
#define outn(a) (out(a),putchar('\n'))
#define I_int  int
	inline I_int read() {
		I_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 * 10 + c - '0'; c = getchar(); }
		return x * f;
	}
	template<typename T>
	bool scan_d(T& ret)
	{
#define p(x) ((x)>='0'&&(x)<='9')
		int c; int sgn;
		if (c = getchar(), c == EOF)return false;
		while (c != '-' && !p(c))c = getchar();
		sgn = (c == '-') ? -1 : 1;
		ret = (c == '-') ? 0 : c - '0';
		while (c = getchar(), p(c))ret = ret * 10 + c - '0';
		ret *= sgn;
		return true;
#undef p
	}
	char F[200];
	template<typename T>
	inline void write(T x) {
		if (x == 0) { putchar('0'); return; }
		auto tmp = x > 0 ? x : -x;
		if (x < 0) putchar('-');
		int cnt = 0;
		while (tmp > 0) {
			F[cnt++] = tmp % 10 + '0';
			tmp /= 10;
		}
		while (cnt > 0) putchar(F[--cnt]);
	}
	void read(char* s, int Arr[], size_t n)
	{
		for (size_t i = 0; i < n; ++i)Arr[i] = strtoll(s, &s, 10);
	}
#undef I_int
}
using namespace io;
#endif // Enable_IO

#ifdef Enable_MATH
template<typename T> constexpr T ModUp(const T& x, ll Mod = mod) { return (x % Mod + Mod) % Mod; }
template<typename T>
constexpr T exgcd(T a, T b, T& x, T& y)
{
	if (a == 0 && b == 0)return -1;
	if (b == 0) { x = 1, y = 0; return a; }
	T d = exgcd(b, a % b, y, x);
	y -= a / b * x;
	return d;
}
template<typename T>
constexpr T inv1(T a, T Mod = mod)
{
	T x = -1, y = -1;
	return exgcd(a, Mod, x, y) == 1 ? (x % Mod + Mod) % Mod : -1;
}
template<typename T>
constexpr T _gcd(T a, T b)
{
	return b ? _gcd(b, a % b) : a;
}
template<typename T>
constexpr T combinator(T n, T m, T Mod = mod - 1)
{
	m = min(m, n - m);
	T ret = 1;
	for (T i = 1; i <= m; ++i)
	{
		ret = ret * (n - m + i) / i % Mod;
	}
	return ret;
}
template<typename T>
constexpr T poww(T a, T b, T Mod = mod)
{
	T ret = 1;
	a %= Mod;
	b %= Mod - 1;
	if (b < 0)b += Mod - 1;
	while (b)
	{
		if (b & 1)ret = ret * a % Mod;
		a = a * a % Mod;
		b >>= 1;
	}
	return ret;
}
constexpr ll inv2(ll a, ll Mod = mod)
{
	return poww(a, Mod - 2, Mod);
}
struct Combinator1
{
	static constexpr int Mod = ::mod;
	static constexpr int Maxn = 1;//::maxn;
	int dp[Maxn][Maxn];//this is C(n,m)%(mod-1)
	constexpr Combinator1() :dp()
	{
		for (int i = 0; i < Maxn; ++i)
		{
			for (int j = 0; j <= i; ++j)
			{
				if (j == 0) dp[i][j] = 1;
				else if (i == j) dp[i][j] = 1;
				else dp[i][j] = (dp[i - 1][j] + dp[i - 1][j - 1]) % (Mod - 1);
			}
		}
	}
};
struct Combinator2
{
	static constexpr int Mod = ::mod;
	static constexpr int Maxn = ::maxn;
	ll Fact[Maxn], Inv_Fact[Maxn];
	constexpr Combinator2(const int n = Maxn - 1) :Fact(), Inv_Fact()
	{
		Fact[0] = 1;
		for (int i = 1; i <= n; ++i)
			Fact[i] = Fact[i - 1] * i % Mod;
		Inv_Fact[0] = 1;
		Inv_Fact[n] = inv2(Fact[n], Mod);
		for (int i = n; i > 1; --i)
			Inv_Fact[i - 1] = Inv_Fact[i] * i % Mod;
	}
	constexpr ll Comb(int a, int b)const
	{
		if (a < 0 || b < 0)return 1;
		return Fact[a] * Inv_Fact[b] % Mod * Inv_Fact[a - b] % Mod;
	}
};
#endif // Enable_MATH

int main()
{
	char s[10];
	while (cin>>s)
	{
		int n = strlen(s);
		for (int i = 0; i < n; ++i)
		{
			if (s[i] == '1')s[i] = '9';
			else if (s[i] == '9')s[i] = '1';
		}
		cout << s << endl;
	}
}

Compiler output:
a.cc: In function 'int main()':
a.cc:181:25: error: 'strlen' was not declared in this scope
  181 |                 int n = strlen(s);
      |                         ^~~~~~
a.cc:4:1: note: 'strlen' is defined in header '<cstring>'; this is probably fixable by adding '#include <cstring>'
    3 | #include <algorithm>
  +++ |+#include <cstring>
    4 | #pragma warning (disable:4996)