https://www.codechef.com/SEPT16/problems/LEXOPAL

fadingpalindrome.cpp

#include <iostream>
#include <cstring>
#include <string>
using namespace std;

// returns true if A is a fading palindome
bool ifp(char* A) {
    int i, n = strlen(A);
    for (i = 0; i < n/2; ++i) {
    	// we need to check the palindome invariant A[i] == A[n-i-1] and
    	// if either characters are '.' we always assume they are the same.
        if ((A[i] != A[n-i-1] && A[i] != '.' && A[n-i-1] != '.')) return false;
    }
    // by this point either the entire string has been checked or there is
    // a single character left in the center of the string (either '.' or something else).
    // this still makes a valid palindrome.
    return true;
}

void test(char* A) {
    int i, n = strlen(A);
    // if it's not a fading palidrome output -1
    if (!ifp(A)) cout << -1;
    else {
        for (i = 0; i < n; ++i) {
            // print the string, but replace '.' with the character on the other
            // half of the string, creating a palindrome.
            // The only case where we print any character from a-z is when the
            // '.' is in the center of the string or characters on both halfs
            // of the strings are '.'s.
            cout <<
                ((A[i] == '.') ?
                    ((i == n/2 || A[n-i-1] == '.') ?
                        'a' : // arbitrary character
                    A[n-i-1]) :
                A[i]);
        }
    }
    cout << '\n';
}

int main() {
    char str[12345+1];
    int t, i;
    cin >> t;
    for (i = 0; i < t; ++i) {
        cin >> str;
        test(str);
    }
}