import bisect
from collections import defaultdict, deque
from itertools import chain

def longest_increasing_subsequence_length(s):
    lis, covers, last_cover_members = deque(), defaultdict(list), []
    for c in s:
        i = bisect.bisect_left(last_cover_members, c)
        covers[i].append(c)
        if i >= len(last_cover_members):
            last_cover_members.append(c)
        else:
            last_cover_members[i] = c

    return len(last_cover_members)

def commonChild(s1, s2):
    indices_sorted_rev = defaultdict(list)
    for i in range(len(s2) - 1, -1, -1):
        indices_sorted_rev[s2[i]].append(i)
    indices_of_s1_chars_in_s2_rev_sorted = list( chain.from_iterable( (indices_sorted_rev[c] for c in s1) ) )
    if not indices_of_s1_chars_in_s2_rev_sorted:
        return 0
    return longest_increasing_subsequence_length( indices_of_s1_chars_in_s2_rev_sorted )