Program to find length of longest substring with character count of at least k in Python

Suppose we have a string s where each characters are sorted and we also have a number k, we have to find the length of the longest substring such that every character occurs at least k times.

So, if the input is like s = "aabccddeeffghij" k = 2, then the output will be 8, as the longest substring here is "ccddeeff" here every character occurs at least 2 times.

To solve this, we will follow these steps −

• Define a function rc() . This will take lst
• c := a map with all characters and their occurrences
• acc := a new list
• ans := 0
• valid := True
• for each x in lst, do
  • if c[x] < k, then
    • valid := False
    • ans := maximum of ans and rc(acc)
    • acc := a new list
  • otherwise,
    • insert x at the end of acc
• if valid true, then
  • return size of acc
• otherwise,
  • ans := maximum of ans and rc(acc)
  • return ans
• From the main method, do the following −
• return rc(a new list by converting each character of s)

Let us see the following implementation to get better understanding −

Example

from collections import Counter
class Solution:
      def solve(self, s, k):
         def rc(lst):
            c = Counter(lst)
            acc = []
            ans = 0
            valid = True
            for x in lst:
               if c[x] < k:
                  valid = False
                  ans = max(ans, rc(acc))
                  acc = []
               else:
                  acc.append(x)
               if valid:
                  return len(acc)
               else:
                  ans = max(ans, rc(acc))
               return ans
            return rc(list(s))
ob = Solution()
s = "aabccddeeffghij"
k = 2
print(ob.solve(s, k))

Input

"aabccddeeffghij", 2

Output

8