Sliding Window

A contiguous window that grows on the right and shrinks on the left.

Core syntax

• Expand / contract — outer for right, inner while shrinks from left.
• Window state — a running sum, or a Counter of the window's contents.

left = 0
window = Counter()
for right, c in enumerate(s):
    window[c] += 1
    while window[c] > 1:
        window[s[left]] -= 1
        left += 1
    best = max(best, right - left + 1)

Watch out

• Window length is right - left + 1.