使用递归查找子序列

Question

I need a program to return True if one string ( s2 ) can be turned into another string ( s1 ) only by removing characters from s2 . So for example if s1 = star and s2 = swewtffsafefr it would return True

I've come up with this so far:

def subsequence(s1, s2, pos=0):
    if pos < len(s1):
        subsequence(s1,s2, pos+1)
    else:
        return 0

I can't figure out how to make it remove characters in s2 that aren't in s1

Answer 1

so you know you want a recursive function subseq(s1,s2)

first you need to come up with your basecases

1. s1 == s2 , for this we will return true
2. s1 is empty , for this we will also return true
3. s2 is empty but s1 is not, we will return false
4. everything else

next you need to put that into code

1. if s1 in s2:return True
2. if not s1: return True
3. if not s2: return False
4. #everything else

for everything else we need to break it down further

1. if s1[0] == s2[0] advance both pointers
2. otherwise only advance s2

.

def subseq(s1,s2):
    if s1 in s2: return True
    if not s1: return True
    if not s2: return False
    #everything else
    if s1[0] == s2[0]: return subseq(s1[1:],s2[1:])
    return subseq(s1,s2[1:])

Answer 2

To remove chars in s2 that are not in s1 is not hard, there are a number of approaches

set_s1 = set([item for item in s1])
set_s2 = set([item for item in s2])

This gets you the unique characters in s1 and s2

Now to identify the values in s2 not in s1

diff = set_s2.difference(set_s1)

Now to remove the characters from s2 in the diff (those in s2 not in s1)

if len(diff) == 0:
    break
else:
    new_s2 = ''
    for c in s2:
        if c not in diff:
            new_s2 = new_s2 + c

Problem you still have is removing duplicates