LeetCode 389. Find the Difference

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.

本题最直观的思路为排序后对s和t逐位进行比较。故得以下代码：

 class Solution
 {
 public:
     char findTheDifference(string s, string t)
     {
         sort(s.begin(), s.end());
         sort(t.begin(), t.end());
         for(int i = ; i < s.size(); i++)
         {
             if(s[i] == t[i])
             {
                 continue;
             }
             else
             {
                 return t[i];
             }
         }
         return t[t.size() - ];
     }
 };

由于排序复杂度过高，于是查询别的解法发现，可以建立一个字母表，s出现一次字母表对应位置+1，t中出现一次对应位置-1。那么只在t中出现的字母的位置为-1。代码如下：

 public char findTheDifference(String s, String t) {
     if(s == null || s.length() == )
         return t.charAt();
     int[] letters = new int[];
     for(int i = ; i < s.length(); i++){
         int sPosition = s.charAt(i) - 'a';
         int tPosition = t.charAt(i) - 'a';
         letters[sPosition]++;
         letters[tPosition]--;
     }
     int tPosition = t.charAt(t.length()-) - 'a';
     letters[tPosition]--;
     char res = 'a';
     for(int i = ; i < letters.length; i++){
         if(letters[i] == -){
             res+= i;
             break;
         }
     }
     return res;
 }