Codility 'Str Symmetry Point' Solution

Martin Kysel · December 7, 2014

Short Problem Definition:

Find a symmetry point of a string, if any.



expected worst-case time complexity is O(length(S))

expected worst-case space complexity is O(1) (not counting the storage required for input arguments).


This problem gave me a lot of headache. It is so trivial I that over-complicated it. I thought that you should find a symmetry point at any possible position, ignoring the residual characters. You would obviously try to maximize the length of this symmetrical sub-array. I was not able to come with any O(S) algorithm for this problem derivation. So just to remind you, this problem is a simple palindrome check. Additionally, you drop all evenly sized strings as their symmetry point is between the indexes.


def solution(S):
    l = len(S)

    if l % 2 == 0:
        return -1

    mid_point = l // 2

    for idx in xrange(0, mid_point):
        if S[idx] != S[l - idx - 1]:
            return -1

    return mid_point

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