Palindrome by swapping only one character

Given a string, the task is to check if the string can be made palindrome by swapping a character only once. 

[NOTE: only one swap and only one character should be swapped with another character]

Examples:  

Input : bbg
Output : true
Explanation: Swap b(1st index) with g.

Input : bdababd
Output : true
Explanation: Swap b(0th index) with d(last index) or
 Swap d(1st index) with b(second index)

Input : gcagac
Output : false

Approach:  

This algorithm was based on a thorough analysis of the behavior and possibility of the forming string palindrome. By this analysis, I got the following conclusions :

1. Firstly, we will be finding the differences in the string that actually prevents it from being a palindrome. 
   1. To do this, We will start from both the ends and comparing one element from each end at a time, whenever it does match we store the values in a separate array as along with this we keep a count on the number of unmatched items.   
2. If the number of unmatched items is more than 2, it is never possible to make it a palindrome string by swapping only one character.   
3. If (number of unmatched items = 2) - it is possible to make the string palindrome if the characters present in first unmatched set are same as the characters present in second unmatched set. (For example : try this out "bdababd").
4.  If (number of unmatched items = 1) 
   1. if (length of string is even) - it is not possible to make a palindrome string out of this. 
   2. if (length of string is odd) - it is possible to make a palindrome string out of this if one of the unmatched character matches with the middle character.
5. If (number of unmatched items = 0) - palindrome is possible if we swap the position of any same characters.

Implementation:

true
true
false

Complexity Analysis:

• Time Complexity : O(n) 
• Auxiliary Space : O(1)