Longest Substring Without Repeating Characters — Guide

• Maintain a window [left, right] that always holds unique characters.
• When a duplicate is found, advance left until the window is valid again.
• Track the maximum window length seen.

1. Use a hash map from character to its most recent index.
2. Expand the right pointer; if the character was seen at or after left, jump left past it.
3. Update the answer with right - left + 1 each step.

def length_of_longest(s):
    last = {}
    left = 0
    best = 0
    for right, c in enumerate(s):
        if c in last and last[c] >= left:
            left = last[c] + 1
        last[c] = right
        best = max(best, right - left + 1)
    return best