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 = 0; i < s.size(); i++)
        {
            if(s[i] == t[i]) 
            {
                continue;
            }
            else
            {
                return t[i];
            }
        }
        return t[t.size() - 1];
    }
};

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() == 0)
        return t.charAt(0);
    int[] letters = new int[26];
    for(int i = 0; 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()-1) - 'a';
    letters[tPosition]--;
    char res = 'a';
    for(int i = 0; i < letters.length; i++){
        if(letters[i] == -1){
            res+= i;
            break; 
        }    
    }
    return res;
}