5. Longest Substring Without Repeating Characters

func longestSubstring(s string) int {
    left := 0
    charToIdx := make(map[byte]int)
    longestLen := 0

    for right := 0; right < len(s); right++ {
        if lastIdx, ok := charToIdx[s[right]]; ok && lastIdx >= left {
            left = lastIdx + 1
        }
        charToIdx[s[right]] = right
        if longestLen < right-left+1 {
            longestLen = right - left + 1
        }
    }

    return longestLen
}