import collections

def longest_nonrepeating_substring(s):
    r = 0
    c = collections.Counter(s[0])
    best_width = 0
    for l in range(len(s)):
        while r < len(s)-1 and c[s[r+1]] < 1:
            r += 1
            c[s[r]] += 1
        best_width = max(best_width, 1+r-l)
        c[s[l]] -= 1
    return best_width


test_cases = (
    ("abcabcbb", 3),
    ("bbbbb", 1),
    ("pwwkew", 3)
)

for s, expected in test_cases:
    print("Input:", s)
    print("Expected output:", expected)
    actual = longest_nonrepeating_substring(s)
    if actual == expected:
        print("   PASSED.")
    else:
        print("   FAILED. Actual output:", actual)