Language: C++

#include<iostream>
#include<algorithm>
#include <iostream> 
#include <algorithm> 
#include<string>
#include<queue>
#include<utility>
#include<fstream>
#include<queue>
#include<set>
#include<stack>
//#include<bits/stdc++.h>
#define ll long long int
#define ull unsigned long long int
#define I(a) scanf("%d",&a)
#define I2(a,b) scanf("%d%d",&a,&b)
#define I3(a,b,c) scanf("%d%d%d",&a,&b,&c)
#define L(a) scanf("%lld",&a)
#define L2(a,b) scanf("%lld%lld",&a,&b)
#define L3(a,b,c) scanf("%lld%lld%lld",&a,&b,&c)
#define PI(a) printf("%d",a)
#define PL(a) printf("%lld",a)
#define PT(t) printf("Case %d: ",t)
#define PB push_back
#define x first
#define y second
#define xx first.first
#define xy first.second
#define yx second.first
#define yy second.second
#define SC scanf
#define PC printf
#define NL printf("\n")
#define SET(a) memset(a,0,sizeof a)
#define SETR(a) memset(a,-1,sizeof a)
#define SZ(a) ((int)a.size())-1
#define f(i,a,b) for(int i=a;i<=b; i++)
#define fr(i,a,b) for(int i=a;i<=b; i++)
#define frr(i,a,b) for(int i=a;i>=b; i--)
#define frv(i,a)  for(int i=0;i<a.size();i++)
#define pi 2.0*acos(0.0)
#define R(a) freopen(a, "r", stdin);
#define W(a) freopen(a, "w", stdout);
#define CB(x) __builtin_popcount(x)
#define STN(a) stringtonumber<ll>(a)
#define lol printf("BUG\n")
#define Endl "\n"
#define mk make_pair
using namespace std;
template <class T> inline T BM(T p, T e, T M)
{
    ll ret = 1;
    for(; e > 0; e >>= 1)
    {
        if(e & 1) ret = (ret * p) % M;
        p = (p * p) % M;
    }
    return (T)ret;
}
template <class T> inline T gcd(T a, T b)
{
    if(b == 0)return a;
    return gcd(b, a % b);
}
template <class T> inline T mdINV(T a, T M)
{
    return BM(a, M - 2, M);
}
template <class T> inline T PW(T p, T e)
{
    ll ret = 1;
    for(; e > 0; e >>= 1)
    {
        if(e & 1) ret = (ret * p);
        p = (p * p);
    }
    return (T)ret;
}
 
 
template <class T>bool ISLEFT(T a, T b, T c)
{
    if(((a.xx - b.xx) * (b.yy - c.yy) - (b.xx - c.xx) * (a.yy - b.yy)) < 0.0)return 1;  //Uporer dike //A,b,c, x okkher ordera sorted
    else return 0;
}
 
 
typedef pair<int,int >P;
//////////////////////////
/////////////////////////
//#include<bits/stdc++.h>
using namespace std;
#define Base1   101LL
#define Base2   10000079LL
#define Base3   10000103LL
#define MOD1    1000000007LL
#define MOD2    1000000009LL
#define MOD3    1000000021LL
#define mx 100000//Number of charter of main string
#define mx1 25000// Number off character of total charcater
#define mx2 500// Number of pattern and number of ch of each pattern

int main(){

    int n;
    I(n);
    ll ar[10][10];
    SET(ar);
    for(int i = 1;i<=n;i++){
        int fi = i % 10;
        int x = i;
        while(x > 9){
            x  /= 10;
        }
        ar[x][fi] ++;

    }
    ll res = 0;
    for(int i = 1;i <= 9;i++){
        for(int j = 1;j<=9;j++){
            res = (res + ar[i][j] * ar[j][i]) ;
        }
    }
    cout<<res<<endl;
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:34:16: error: 'memset' was not declared in this scope
   34 | #define SET(a) memset(a,0,sizeof a)
      |                ^~~~~~
a.cc:108:5: note: in expansion of macro 'SET'
  108 |     SET(ar);
      |     ^~~
a.cc:12:1: note: 'memset' is defined in header '<cstring>'; this is probably fixable by adding '#include <cstring>'
   11 | #include<stack>
  +++ |+#include <cstring>
   12 | //#include<bits/stdc++.h>