Finding the Longest Substring Without Repeating Characters via Sliding Window

The task is to determine the maximum‑length substring of a given string that contains no repeated characters.

The solution relies on a sliding‑window approach: two pointers, left and right, both start at 0. right moves forward one character at a time, while the window’s contents are tracked to ensure all characters are unique. When a duplicate appears, left advances to discard characters until the window becomes valid again.

Typical pseudocode:

define left = 0, right = 0
define win to store results
while right is within bounds:
    add s[right] to win
    right++
    while win does not satisfy uniqueness:
        remove s[left] from win
        left++

The core check is whether any character between left and right repeats.

Complete Go code:

func lengthOfLongestSubstring(s string) int {
    // sliding window boundaries
    left, right := 0, 0

    // count of characters in the window
    win := make(map[byte]int)

    result := 0
    max := func(a, b int) int {
        if a > b {
            return a
        }
        return b
    }
    for right < len(s) {
        ch := s[right] // current character
        right++
        win[ch]++ // increment count

        for win[ch] > 1 { // duplicate found, shrink window
            d := s[left]
            left++
            win[d]--
        }

        // window is valid, update result
        result = max(result, right-left)
    }
    return result
}