Here is my solution. so before code this. i am trying to imagine how the palindrome works. and i found that. you only need to check first wrong character (first wrong from left, and first wrong from right)

then what i do next ? I try to remove wrong character in left, check whether it is already palindrome. if no then i try to remove wrong character in right. if it is still not palindrome then there is no solution.

public static int palindromeIndex(String s) {
// Write your code here
    int leftWrongIndex = -1;
    int rightWrongIndex = -1;
    int sLength = s.length();
    for(int i=0; i < sLength/2; i++) {
        if(s.charAt(i) != s.charAt(sLength - 1 - i)) {
            leftWrongIndex = i;
            rightWrongIndex = sLength -1 -i;
            break;
        }
    }
    if(leftWrongIndex == -1 && rightWrongIndex == -1) {
        return -1;
    }

    if (checkPalindrome(s.substring(leftWrongIndex + 1, rightWrongIndex + 1))) {
        return leftWrongIndex;
    } else if (checkPalindrome(s.substring(leftWrongIndex, rightWrongIndex))) {
        return rightWrongIndex;
    }

    return -1;
}

public static boolean checkPalindrome(String s) {
    int sLength = s.length();
    for(int i=0; i< s.length() /2 ;i ++) {
        if(s.charAt(i) != s.charAt(sLength - 1 - i)) {
            return false;
        }
    }
    return true;
}