Sliding Window

Expand right, shrink left

For "longest/shortest/best contiguous subarray or substring" problems: move right to include a new element; while the window breaks a constraint, move left to drop elements. Each index enters and leaves once — O(n).

Longest substring without repeats

Track the last index of each character; jump left past any repeat.

def longest_unique(s):
    last = {}
    left = 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

Output:

longest_unique("abcabcbb") -> 3   ("abc")

Spotting it

Keywords: "contiguous", "subarray/substring", "at most K", "sum equals", "longest/shortest". Fixed-size window is even simpler — slide by one and adjust the running total.