LeetCode_ 6124_ The first letter that appears twice

Topic link

• https://leetcode.cn/problems/first-letter-to-appear-twice/

Title Description

Give you a string of lowercase English letters s , Please find out and return to the first occurrence two The letter of .

Be careful ：

If a Of The second time Appearance ratio b Of The second time Appear higher in the string , It is considered as a letter a In the letters b Twice before .
s Contains at least one letter that appears twice .

Example 1：

 Input ：s = "abccbaacz"
 Output ："c"
 explain ：
 Letter  'a'  In subscript  0 、5  and  6  Appears at .
 Letter  'b'  In subscript  1  and  4  Appears at .
 Letter  'c'  In subscript  2 、3  and  7  Appears at .
 Letter  'z'  In subscript  8  Appears at .
 Letter  'c'  Is the first letter that appears twice , Because of all the letters ,'c'  The subscript of the second occurrence is the smallest .

Example 2：

 Input ：s = "abcdd"
 Output ："d"
 explain ：
 Only letters  'd'  Twice , So back  'd' .

Tips ：

• 2 <= s.length <= 100
• s It's made up of lowercase letters
• s Contains at least one repeating letter

Their thinking

HashSet To do it

AC Code

class Solution {
    
    public char repeatedCharacter(String s) {
    
        HashSet<Character> set = new HashSet<>();
        for (int i = 0; i < s.length(); i++) {
    
            char c = s.charAt(i);
            if (set.contains(c)) {
    
               return c;
            } else {
    
                set.add(c);
            }
        }
        return ' ';
    }
}