Given a string s consisting of lowercase English letters, return the first letter to appear twice.
Note:
A letter a appears twice before another letter b if the second occurrence of a is before the second occurrence of b.
s will contain at least one letter that appears twice.
Example 1:
Input: s = "abccbaacz"
Output: "c"
Explanation:
The letter 'a' appears on the indexes 0, 5 and 6.
The letter 'b' appears on the indexes 1 and 4.
The letter 'c' appears on the indexes 2, 3 and 7.
The letter 'z' appears on the index 8.
The letter 'c' is the first letter to appear twice, because out of all the letters the index of its second occurrence is the smallest.
Example 2:
Input: s = "abcdd"
Output: "d"
Explanation:
The only letter that appears twice is 'd' so we return 'd'.
Constraints:
2 <= s.length <= 100
s consists of lowercase English letters.
s has at least one repeated letter.
Solutions
Solution 1: Array or Hash Table
We traverse the string $s$, using an array or hash table cnt to record the occurrence of each letter. When a letter appears twice, we return that letter.
The time complexity is $O(n)$ and the space complexity is $O(C)$. Here, $n$ is the length of the string $s$, and $C$ is the size of the character set. In this problem, $C = 26$.
We can also use an integer mask to record whether each letter has appeared, where the $i$-th bit of mask indicates whether the $i$-th letter has appeared. When a letter appears twice, we return that letter.
The time complexity is $O(n)$ and the space complexity is $O(1)$. Here, $n$ is the length of the string $s$.