Let \(|S|\) denote the length of \(S\). Then the value \(k\) in the condition of \(T_i\) is \(k=\dfrac{m-|S_i|}{2}\).

Possible ways to find \(T_i\) include “prepending and appending a . to the head and tail of \(S_i\), \(k\) times,” and “use the \(k\)-time repetition of ..”

Sample code (C++)

#include <bits/stdc++.h>
using namespace std;
int main() {
    int N;
    cin >> N;
    vector<string> S(N);
    for (int i = 0; i < N; i++) cin >> S[i];
    int m = 0;
    for (int i = 0; i < N; i++) m = max(m, (int)S[i].size());
    for (int i = 0; i < N; i++) {
        int k = (m - S[i].size()) / 2;
        string T = string(k, '.') + S[i] + string(k, '.');
        cout << T << "\n";
    }
}

Sample code (Python)

N = int(input())
S = [input() for _ in range(N)]
m = max(len(s) for s in S)
for s in S:
    k = (m - len(s)) // 2
    t = ("." * k) + s + ("." * k)
    print(t)