Description： 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.
 

題意：給定兩個字串s和t,其中字串t是由s中的所有字元隨機組合後再新增一個隨機字元再任意位置形成的，現在要求找出字串t中新增的這個隨機字元；

解法：由題意我們知道字串s和t僅僅相差了一個字元，因此我們可以用雜湊表分別儲存字串s和t中每個字元出現的次數，之後我們遍歷字串t中的所有鍵（即t中的不同字元），找出與字串s中相應字元次數不相同的那個，就是隨機新增的字元；

Java

class Solution {
    public char findTheDifference(String s, String t) {
        Map<Character, Integer> cntS = new HashMap<>();
        Map<Character, Integer> cntT = new HashMap<>();
        char result = ' ';
        for (int i = 0; i < t.length(); i++) {
            if (i < s.length()) {
                cntS.put(s.charAt(i), cntS.getOrDefault(s.charAt(i), 0) + 1);
            }
            cntT.put(t.charAt(i), cntT.getOrDefault(t.charAt(i), 0) + 1);
        }
        for (Character key : cntT.keySet()) {
            if (cntT.get(key) != cntS.getOrDefault(key, 0)) {
                result = key;
                break;
            }
        }
        return result;
    }
}