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Input: A string of "a", "b" and "c".
Output: Return true if exactly one letter is differnet from adjacent letters.
Otherwise, return false.
For example, in string "babbcc", first a, and first b are different from neighbours. Thus return false.
Constraint: 1 <= Inout length <= 7

Example:
Input: aabcc
Output: true
Input: abbcc
Output: true
Input: babbcc
Output: false