问题：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.
　思路：使用hashmap，计算s中每个字符出现的次数。再遍历字符串t，每遇到一个字符，将map中的值减1。如果字符不出现在map，或者计数为0，就是后来加入的字符。
　

public char findTheDifference(String s, String t) {Map<Character,Integer> map = new HashMap<Character,Integer>();for (int i = 0; i < s.length(); i++) {if(map.get(s.charAt(i))==null){map.put(s.charAt(i), 1);}else{map.put(s.charAt(i), map.get(s.charAt(i))+1);}}for (int i = 0; i < t.length(); i++) {if (!map.keySet().contains(t.charAt(i)) || map.get(t.charAt(i))<=0) {return t.charAt(i);}else{map.put(t.charAt(i), map.get(t.charAt(i))-1);}}return t.charAt(0);}

收获：1 用异或。a^b^a=b，a^b^b=a，a^b^a^b=0。交换两个int，不用第三个空间的做法是：a=a^b，b=a^b，a=a^b。

参考资料
1 题目
2 讨论