How to find longest Substring Without Repeating Characters ?

[Question]: Given a string s, find the length of the longest substring without repeating characters.

Example 1:

Input: s = "abcabxy"
Output: 5
Explanation: The answer is "abcxy", with the length of 5

func lengthOfLongestSubstring(_ s: String) -> Int {
    var longest = 0, startIndex = 0
    var charMap: [Character: Int] = [:]
    for (index, char) in s.enumerated() {
        if let foundIndex = charMap[char] {
            startIndex = max(foundIndex + 1, startIndex)
        }
        longest = max(longest, index - startIndex + 1)
        charMap[char] = index
    }
    return longest
}

let longestSubStrInput = "abcabxy"
let outputOfLongest = lengthOfLongestSubstring(longestSubStrInput)
//print(" output of longest substring without repeating chars", outputOfLongest) // OP->5