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389. Find the Difference

Description

You are given two strings s and t.

String t is generated by random shuffling string s and then add one more letter at a random position.

Return the letter that was added to t.

 

Example 1:

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

Example 2:

Input: s = "", t = "y"
Output: "y"

 

Constraints:

  • 0 <= s.length <= 1000
  • t.length == s.length + 1
  • s and t consist of lowercase English letters.

Solutions

Solution 1: Counting

We can use a hash table or array $cnt$ to count the occurrence of each character in string $s$, then traverse string $t$. For each character, we subtract its occurrence in $cnt$. If the corresponding count is negative, it means that the occurrence of this character in $t$ is greater than in $s$, so this character is the added character.

The time complexity is $O(n)$, and the space complexity is $O(|\Sigma|)$, where $n$ is the length of the string, and $\Sigma$ represents the character set. Here the character set is all lowercase letters, so $|\Sigma|=26$.

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class Solution:
    def findTheDifference(self, s: str, t: str) -> str:
        cnt = Counter(s)
        for c in t:
            cnt[c] -= 1
            if cnt[c] < 0:
                return c

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class Solution {
    public char findTheDifference(String s, String t) {
        int[] cnt = new int[26];
        for (int i = 0; i < s.length(); ++i) {
            ++cnt[s.charAt(i) - 'a'];
        }
        for (int i = 0;; ++i) {
            if (--cnt[t.charAt(i) - 'a'] < 0) {
                return t.charAt(i);
            }
        }
    }
}

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class Solution {
public:
    char findTheDifference(string s, string t) {
        int cnt[26]{};
        for (char& c : s) {
            ++cnt[c - 'a'];
        }
        for (char& c : t) {
            if (--cnt[c - 'a'] < 0) {
                return c;
            }
        }
        return ' ';
    }
};

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func findTheDifference(s, t string) byte {
    cnt := [26]int{}
    for _, ch := range s {
        cnt[ch-'a']++
    }
    for i := 0; ; i++ {
        ch := t[i]
        cnt[ch-'a']--
        if cnt[ch-'a'] < 0 {
            return ch
        }
    }
}

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function findTheDifference(s: string, t: string): string {
    const cnt: number[] = Array(26).fill(0);
    for (const c of s) {
        ++cnt[c.charCodeAt(0) - 'a'.charCodeAt(0)];
    }
    for (const c of t) {
        --cnt[c.charCodeAt(0) - 'a'.charCodeAt(0)];
    }
    for (let i = 0; ; ++i) {
        if (cnt[i] < 0) {
            return String.fromCharCode(i + 'a'.charCodeAt(0));
        }
    }
}

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impl Solution {
    pub fn find_the_difference(s: String, t: String) -> char {
        let s = s.as_bytes();
        let t = t.as_bytes();
        let n = s.len();
        let mut count = [