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painless
Find a symmetry point of a string, if any.
Programming language: Spoken language:

Write a function:

class Solution { public int solution(String S); }

that, given a string S, returns the index (counting from 0) of a character such that the part of the string to the left of that character is a reversal of the part of the string to its right. The function should return −1 if no such index exists.

Note: reversing an empty string (i.e. a string whose length is zero) gives an empty string.

For example, given a string:

"racecar"

the function should return 3, because the substring to the left of the character "e" at index 3 is "rac", and the one to the right is "car".

Given a string:

"x"

the function should return 0, because both substrings are empty.

Write an efficient algorithm for the following assumptions:

  • the length of S is within the range [0..2,000,000].
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