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Find the smallest window in a string containing all characters of another string in Python
Suppose we have two strings s1 and s2, we have to find the smallest substring in s1 such that all characters of s2 will be used efficiently.
So, if the input is like s1 = "I am a student", s2 = "mdn", then the output will be "m a studen"
To solve this, we will follow these steps −
N := 26
str_len := size of main_str, patt_len := size of pattern
if str_len < patt_len, then
return None
hash_pat := an array of size N and fill with 0
hash_str := an array of size N and fill with 0
for i in range 0 to patt_len, do
hash_pat[ASCII of(pattern[i]) ] := hash_pat[ASCII of(pattern[i]) ] + 1
start := 0, start_index := -1, min_len := inf
count := 0
for j in range 0 to str_len, do
hash_str[ASCII of(main_str[j]) ] := hash_str[ASCII of(main_str[j]) ] + 1
if hash_pat[ASCII of(main_str[j]) ] is not same as 0 and hash_str[ASCII of(main_str[j]) ] <= hash_pat[ASCII of(main_str[j]) ], then
count := count + 1
if count is same as patt_len, then
while hash_str[ASCII of(main_str[start]) ] > hash_pat[ASCII of(main_str[start]) ] or hash_pat[ASCII of(main_str[start]) ] is same as 0, do
if hash_str[ASCII of(main_str[start])] > hash_pat[ASCII of(main_str[start])], then
hash_str[ASCII of(main_str[start]) ] := hash_str[ASCII of(main_str[start]) ] - 1
start := start + 1
len_window := j - start + 1
if min_len > len_window, then
min_len := len_window
start_index := start
if start_index is same as -1, then
return None
return substring of main_str[from index start_index to start_index + min_len]
Example
Let us see the following implementation to get better understanding −
N = 256
def get_pattern(main_str, pattern):
str_len = len(main_str)
patt_len = len(pattern)
if str_len < patt_len:
return None
hash_pat = [0] * N
hash_str = [0] * N
for i in range(0, patt_len):
hash_pat[ord(pattern[i])] += 1
start, start_index, min_len = 0, -1, float('inf')
count = 0
for j in range(0, str_len):
hash_str[ord(main_str[j])] += 1
if (hash_pat[ord(main_str[j])] != 0 and hash_str[ord(main_str[j])] <= hash_pat[ord(main_str[j])]):
count += 1
if count == patt_len:
while (hash_str[ord(main_str[start])] > hash_pat[ord(main_str[start])] or hash_pat[ord(main_str[start])] == 0):
if (hash_str[ord(main_str[start])] > hash_pat[ord(main_str[start])]):
hash_str[ord(main_str[start])] -= 1
start += 1
len_window = j - start + 1
if min_len > len_window:
min_len = len_window
start_index = start
if start_index == -1:
return None
return main_str[start_index : start_index + min_len]
main_str = "I am a student"
pattern = "mdn"
print(get_pattern(main_str, pattern))Input
"I am a student", "mdn"
Output
m a studen
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