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;
}

Here is my C++ solution

// helper function
bool isPalindrome ( const string & str)
{
    for (int i = 0; i < str.length() / 2; i++)
    { 
        if (str[i] != str[str.length() - i - 1])
            return false;
    }
    return true;
} 

int  palindromeIndex(string& s)
{
  int backward = 0, forward = s.size() - 1;

  bool bPallindrome = true;

  while (backward < forward) 
  {
      if (s[backward] != s[forward]) 
      {
          bPallindrome = false;
          break;
      }
      backward++;
      forward--;
  }
  int result = -1;

  if (!bPallindrome)
  {
    result = isPalindrome(s.erase(forward, 1)) ? forward: backward;
  }
  return result;

}

Can't for the life of me figure out what's wrong with my solution - it passes 12/15 test cases.

def palindromeIndex(s):
        for i in range(len(s) // 2):
                j = len(s) - 1 - i
                if (s[i] == s[j]):
                        continue
                if (s[i] == s[j - 1]):
                        return j
                if (s[i + 1] == s[j]):
                        return i
        return -1

Python Complexity:O(n)

def checkPalindrome(s):
    for i in range(len(s) // 2):
        if s[i] != s[len(s) - i - 1]:
            return False
    return True

def palindromeIndex(s):
    idx = -1
    # s is palindrome
    if checkPalindrome(s):
        return -1
        
    # find the first character index that does not match
    for i in range(len(s) // 2):
        if s[i] == s[len(s) - i - 1]:
            continue
        else:
            idx = i
            break
    
    # drop the first character
    sub_s_1 = s[(idx+1) : (len(s) - idx)]
    if checkPalindrome(sub_s_1):
        return idx
    
    # drop the last character
    sub_s_2 = s[idx : (len(s) - idx - 1)]
    if checkPalindrome(sub_s_2):
        return len(s) - idx - 1

    return -1