import Foundation
func longestSubstring(_ s: String) -> String {
var start = 0
var maxLength = 0
var longestSubstr = ""
var charIndexMap = [Character: Int]()
for (end, char) in s.enumerated() {
// If the character is already in the map and within the current window
if let index = charIndexMap[char], index >= start {
start = index + 1 // Move the start pointer to avoid duplicates
}
// Update the character's latest index
charIndexMap[char] = end
// Check if the current substring is the longest
let currentLength = end - start + 1
if currentLength > maxLength {
maxLength = currentLength
longestSubstr = String(s[s.index(s.startIndex, offsetBy: start)...s.index(s.startIndex, offsetBy: end)])
}
}
return longestSubstr
}
print(longestSubstring("abcabcbb")) // Output: "abc"
print(longestSubstring("bbbbb")) // Output: "b"
print(longestSubstring("xwwwqfwwxqwyq")) // Output: "xqwy"
/*
run:
abc
b
xqwy
*/
