function 返回第一个可能的 substring 包含 substring 的所有元素

Question

How can I modify the myStr function to return the first possible substring (traversing from left) containing all the elements of str2 . For example: given str1 = 'xBxxAxxCxxAxCxxBxxxAxCxBxxxAxxBxCx' and str2 = "ABC" , the function should return "BxxAxxC" .

str1 = input("Enter the first string: ")
str2 = input("Enter the second string to check if the characters exist in the first string: ")

def myStr(str1, str2):
    
    if all(i in str1 for i in str2):
        return True
    else:     
        return False

myStr(str1, str2)

Answer 1

If str2 doesn't contain any duplicates letters, you can map its characters to their first positions in str1 and get the range between the minimum and maximum positions:

for example:

str1   = 'xBxxAxxCxxAxCxxBxxxAxCxBxxxAxxBxCx'
str2   = "ABC"

pos    = [str1.find(c) for c in str2]
result = str1[min(pos):max(pos)+1]

print(result) # BxxAxxC

If not all characters of str2 are present in str1, this will produce an empty string

If str2 CAN contain duplicate letters, then the list of positions (pos) needs to include multiple positions for repeated characters:

pos = [i for r in [list(str2)] for i,c in enumerate(str1) 
       if c in r and not r.remove(c)]

Answer 2

Assuming that left-most characters that are not in str2 should be excluded , you could do the following:

• Create a set for the characters in str2
• Iterate the characters from str1 and remove them from this set.
• Once the set is empty, you know that the condition is fulfilled.
• To exclude the left most characters, keep track which was the first index where there was a match with a character in the set.

def myStr(str1, str2):
    todo = set(str2)
    start = -1
    for i, ch in enumerate(str1):
        if not todo:
            return str1[start:i]
        if ch in todo:
            todo.discard(ch)
            if start < 0:
                start = i

Answer 3

You can use set operations, for each prefix check that all letter are in with

set(str2) - set(str1[:1])

set(str2) - set('xB')       # {'C', 'A'}
set(str2) - set('xBxxA')    # {'C'}
set(str2) - set('xBxxAxxC') # set()

def myStr(str1, str2):
    chars_to_have = set(str2)
    i = 0
    while chars_to_have - set(str1[:i]) != set() and i < len(str1):
        i += 1
    return str1[:i]