389. Find the Difference

题目

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.

Example:
Input:
s = "abcd"
t = "abcde"

Output:
e

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

解题思路

见程序注释