repeated-substring-pattern.py

from __future__ import print_function
# Time:  O(n)
# Space: O(n)

# Given a non-empty string check if it can be constructed by taking a substring of it
# and appending multiple copies of the substring together.
# You may assume the given string consists of lowercase English letters only and its length will not exceed 10000.
#
# Example 1:
# Input: "abab"
#
# Output: True
#
# Explanation: It's the substring "ab" twice.
# Example 2:
# Input: "aba"
#
# Output: False
# Example 3:
# Input: "abcabcabcabc"
#
# Output: True
#
# Explanation: It's the substring "abc" four times. (And the substring "abcabc" twice.)

# KMP solution.


class Solution(object):
    def repeatedSubstringPattern(self, str):
        """
        :type str: str
        :rtype: bool
        """
        def getPrefix(pattern):
            prefix = [-1] * len(pattern)
            j = -1
            for i in xrange(1, len(pattern)):
                while j > -1 and pattern[j + 1] != pattern[i]:
                    j = prefix[j]
                if pattern[j + 1] == pattern[i]:
                    j += 1
                prefix[i] = j
            return prefix

        prefix = getPrefix(str)
        return prefix[-1] != -1 and \
               (prefix[-1] + 1) % (len(str) - prefix[-1] - 1) == 0

    def repeatedSubstringPattern2(self, str):
        """
        :type str: str
        :rtype: bool
        """
        if not str:
            return False

        ss = (str + str)[1:-1]
        print(ss)
        return ss.find(str) != -1