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:

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Input:
s = "abcd"
t = "abcde"
Output:
e
Explanation:
'e' is the letter that was added.

方法1：HashMap

最直观的做法是统计s中每个字符出现的次数，然后在扫描t，把相应字符的counter–， 如果减过之后，counter == -1，那就说明这个字就是所要找的字符

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public char findTheDifference(String s, String t) {
    int[] count = new int[26];
    for(char c : s.toCharArray()) {
        count[c - 'a']++;
    }
    for(char c : t.toCharArray()) {
        if(--count[c - 'a'] < 0)
            return c;
    }
    return ' ';
}

方法2: bit manipulation

这个题目很像136. Single Number, 如果把字符看成数字，s和t中除了所求字符外，所有的字符都出现了两次。所求字符就是Single Number.

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public char findTheDifference(String s, String t) {
    char result = 0;
    for (char c : s.toCharArray())
        result ^= c;
    for (char c : t.toCharArray())
        result ^= c;
    return result;
}