find_difference.py

"""
Given two strings s and t which consist of only lowercase letters.
String t is generated by random shuffling string s and then add one more letter
at a random position. Find the letter that was added in t.

For example:
Input:
s = "abcd"
t = "abecd"
Output: 'e'

Explanation:
'e' is the letter that was added.
"""

"""
We use the characteristic equation of XOR.
A xor B xor C = A xor C xor B
If A == C, then A xor C = 0
and then, B xor 0 =  B
"""
def find_difference(s, t):
    ret = 0
    for ch in s + t:
        # ord(ch) return an integer representing the Unicode code point of that character
        ret = ret ^ ord(ch)
    # chr(i) Return the string representing a character whose Unicode code point is the integer i
    return chr(ret)