The first unique character in a string java

Given a string, find its first unique character and return its index. If it does not exist, -1 is returned.

Example:

s = "leetcode"
returns 0

s = "loveleetcode"
returns 2

Source: LeetCode (LeetCode)
Link: https://leetcode-cn.com/problems/first-unique-character-in-a-string
Copyright is owned by LeetCode . For commercial reprints, please contact the official authorization. For non-commercial reprints, please indicate the source.

class Solution {
    
    
    public int firstUniqChar(String s) {
    
    
        int[] mark = new int[26];
        for(int i=0;i<s.length();i++)
        {
    
    
            char ch = s.charAt(i);
            mark[ch-'a']++;
        }
        for(int i=0;i<s.length();i++)
        {
    
    
            char ch = s.charAt(i);
            if(mark[ch-'a']==1)
            {
    
    
                return i;
            }
        }
        return -1;
    }
}