Check if a given string can be a palindrome by altering some character without using a library function
The above looks to be more work than is necessary. Assuming that we are working with all lower case chars, the following should work.

We match the chars at each end, moving inwards on each cycle. Normally, if any do not match, we consider it a fail and break out. If we are allowed a one pair mismatch (i.e. change one character), then we need to keep track of the count of mismatches and if we have 0 or 1 (less than 2) we consider it to be a palindrome.

private static boolean isAlmostPalindrome(String str){
    int diffCount = 0;
    int left = 0;
    int right = str.length() -1;

    while(right&gt;left){
        if (str.charAt(right--)!= str.charAt(left++)){
            diffCount++;
        }
    }
    return diffCount &lt;2;
}