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 ordch return an integer representing the Unicode code point of that character chri Return the string representing a character whose Unicode code point is the integer i
"""
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)
