LeetCode - 389. Find the Difference

Q：

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.

A：

class Solution(object):
def findTheDifference(self, s, t):
"""
:type s: str
:type t: str
:rtype: str
"""
sDic = {}
for ch in s:
sDic[ch] = sDic.get(ch, 0) + 1
for ch in t:
if sDic.get(ch, 0) == 0:
return ch
else:
sDic[ch] -= 1

主要思路：

先for遍历字符串s，统计每个字母出现的次数（sDic.get(ch, 0)当字典中无该键时，自动创建并取值为0）；

之后便是遍历字符串t，若遍历到的字符并不在字典当中，也就是新添加的字母，这时给它默认为0（sDic.get(ch, 0)），则值为0时返回此字符；

若遍历的字母在字典中，则在其值上减去1，若新添加的字母与之前字符串中有重复，则会出现该字符值不断减1后到达0，还有一个该字母，这时因其值已经等于0，返回。