PalindromeIndex.fs

module PalindromeIndex
//Hackertank problem
//Palindrome Index
// Given a string, , of lowercase letters, determine the index of the character whose removal will make a
// palindrome. If is already a palindrome or no such character exists, then print . There will always be a
// valid solution, and any correct answer is acceptable. For example, if "bcbc", we can either remove 'b'
// at index or 'c' at index .
// Input Format
// The first line contains an integer, , denoting the number of test cases.
// Each line of the subsequent lines (where ) describes a test case in the form of a single string,
// .
// Constraints
// All characters are lowercase English letters.
// Output Format
// Print an integer denoting the zero-indexed position of the character that makes not a palindrome; if is
// already a palindrome or no such character exists, print .
// Sample Input
// 3
// aaab
// baa
// aaa
// Sample Output
// 3
// 0
// -1
// Explanation
// Test Case 1: "aaab"
// Removing 'b' at index results in a palindrome, so we print on a new line.
// Test Case 2: "baa"
// Removing 'b' at index results in a palindrome, so we print on a new line.
// Test Case 3: "aaa"
// This string is already a palindrome, so we print ; however, , , and are also all acceptable answers, as
// the string will still be a palindrome if any one of the characters at those indices are removed.

let isPalindrome' (str: string) = 
  let rec loop i = 
    if i > str.Length/2 then true
    elif str.[i] <> str.[str.Length - 1 - i] then false 
    else loop (i + 1)
  loop 0 

let isPalindromeFromIndex left right (str: string): bool = 
  isPalindrome' (str.Substring(left, (right - left) + 1))   

let removeElementToFormPalindrome' (str: string) =
  let rec loop i = 
    if i > str.Length/2 then -1 
    elif str.[i] <> str.[str.Length - 1 - i] && (isPalindromeFromIndex (i+1) (str.Length - 1 - i) str) then i 
    elif str.[i] <> str.[str.Length - 1 - i] && (isPalindromeFromIndex i (str.Length - 2 - i) str) then (str.Length - 1 - i) 
    else loop (i + 1)
  loop 0 


[<EntryPoint>]
let main argv =
  let numStrings = int (System.Console.ReadLine())
  [
    for i in 1..numStrings do
     yield System.Console.ReadLine() |> removeElementToFormPalindrome'
  ]
  |> List.iter (printfn "%A") 
  0