Chapter 1 Arrays and Strings

Problem 1.8: Assume you have a method isSubString which checks if one word is a substring of another. Given two strings, s1 and s2, write code to check if s2 is a rotation of s1 using only one call to isSubString (i.e. "waterbottle" is a rotation of "erbottlewat")

It's very important that how you understand the concept "rotation". In my eyes, you just shift the word and the letters go out of range will appear at the other end. This kind of understanding is not helpful and I didn't know why I need the isSubString to solve the problem.

An ACM guy told me his understanding and it is a great one: you just concatenate the string with itself on both side, and then pick a segment from the new string. If length of the segment is the same as original string (before concatenation), you will get a rotation of original string.

Based on the better understanding, the algorith has just two step:
1) Check whether the lengths of two string are the same.
2) Concatenate one string and check whether the other one is a substring of the concatenation.

import string

def isSubstring(substr, str):
    for i in range(0, len(str)-len(substr)+1):
        if substr[0] == str[i]:
            is_substr = True
            for j in range(0, len(substr)):
                if substr[j] != str[i+j]:
                    is_substr = False
            if True == is_substr:
                return True
    return False

def is_rotation(str1, str2):
    # 1. Check the length
    if len(str1) != len(str2):
        return False
    
    # 2. Concatenate str1
    concatenation_str = str1+str1
    print concatenation_str

    # 3. Check whether str2 is a substring of
    # the concatenation
    return isSubstring(str2, concatenation_str)
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