First Unique Character in a String

Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.

Example 1:

Input: s = "leetcode" Output: 0 Example 2:

Input: s = "loveleetcode" Output: 2 Example 3:

Input: s = "aabb" Output: -1

Constraints:

1 <= s.length <= 105 s consists of only lowercase English letters.

public class Solution {
    public int FirstUniqChar(string s) {
        if (s != null)
        {
            for (int i = 0; i < s.Length; i++)
            {
                var isDuplicate = false;
                for (int j = 0; j < s.Length; j++)
                {
                    if (s[i] == s[j] && i != j)
                    {
                        isDuplicate = true;
                        break;
                    }

                }
                if (!isDuplicate)
                {
                    return i;
                }
            }
        }
        return -1;
    }
}