最长的不重复子串有多长

字符串是 abcabcbb。运行下面这段程序：

def longest_unique(s):
    seen = {}
    lo = 0
    best = 0
    for hi in range(len(s)):
        ch = s[hi]
        if ch in seen and seen[ch] >= lo:
            lo = seen[ch] + 1
        seen[ch] = hi
        if hi - lo + 1 > best:
            best = hi - lo + 1
    return best

print(longest_unique("abcabcbb"))