t = raw_input()
p = raw_input()
aa = [int(x) - 1 for x in raw_input().split()]
l = len(p)
r = lt = len(t)
while l < r:
m = (l + r) // 2
sa = set(aa[:lt-m])
it = (a for i, a in enumerate(t) if i not in sa)
if all(c in it for c in p):
r = m
else:
l = m + 1
print(lt - l)
In particular, how does the line
if all(c in it for c in p):
work? How is it able to ensure that p is a subsequence of it by a simple containment check over all the letters of p? I expected it to output 2 (instead of 1, which is the correct answer) for the following case, but it worked correctly.
abab
ab
1 4 3 2
