Longest Substring Without Repeating Characters

July 26, 2025

Question

Given a string s, find the length of the longest substring without repeating characters.

Example

Input: "pwwkew"

Output: 3

Explanation: The answer is "wke", with the length of 3.

My Notes

We can use a sliding window to keep track of non-repeating characters.
Two pointers: one expanding (right), one shrinking (left) when a duplicate is found.
Use a set (map) to track which characters are inside the window.
Update the result as we go.

Brute Force

package main

import (
    "fmt"
)
                
func lengthOfLongestSubstring(s string) int {
    if len(s) < 2 {
        return len(s)
    }
    result := 0

    for i := range s {
        lookup := make(map[string]struct{})

        for j := range s[i:] {
            if _, ok := lookup[string(s[i+j])]; ok {
                break
            }
            if j+1 > result {
                result = j + 1
            }
            lookup[string(s[i+j])] = struct{}{}
        }
    }

    return result
}
                
func main() {
    result := lengthOfLongestSubstring("pwwkew")
    fmt.Println(result)
}

Optimized Sliding Window

package main

import (
    "fmt"
)
func lengthOfLongestSubstring(s string) int {
    result := 0
    l := 0
    lookup := make(map[byte]struct{})

    for r := 0; r < len(s); r++ {
        // If s[r] is in the map, shrink the window from the left
        for {
            if _, ok := lookup[s[r]]; !ok {
                break
            }
            delete(lookup, s[l])
            l++
        }

        // Add current character to the set
        lookup[s[r]] = struct{}{}

        // Update result
        if r-l+1 > result {
            result = r - l + 1
        }
    }

    return result
}
                
                
func main() {
    result := lengthOfLongestSubstring("pwwkew")
    fmt.Println(result)
}