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+### https://theweeklychallenge.org/blog/perl-weekly-challenge-247/
+"""
+
+Task 2: Most Frequent Letter Pair
+
+Submitted by: [55]Jorg Sommrey
+     __________________________________________________________________
+
+   You are given a string S of lower case letters 'a'..'z'.
+
+   Write a script that finds the pair of consecutive letters in S that
+   appears most frequently. If there is more than one such pair, chose the
+   one that is the lexicographically first.
+
+Example 1
+
+Input: $s = 'abcdbca'
+Output: 'bc'
+
+'bc' appears twice in `$s`
+
+Example 2
+
+Input: $s = 'cdeabeabfcdfabgcd'
+Output: 'ab'
+
+'ab' and 'cd' both appear three times in $s and 'ab' is lexicographically smalle
+r than 'cd'.
+     __________________________________________________________________
+
+   Last date to submit the solution 23:59 (UK Time) Sunday 17th December
+   2023.
+     __________________________________________________________________
+
+SO WHAT DO YOU THINK ?
+"""
+### solution by pokgopun@gmail.com
+
+def mflp(string: str):
+    l = len(string)
+    if l <= 2:
+        return ""
+    dct = dict()
+    for i in range(l-1):
+        pair = string[i:i+2]
+        if dct.get(pair) is None:
+            dct[pair] = 1
+        else:
+            dct[pair] += 1
+    lst = list((-dct[x],x) for x in dct.keys())
+    lst.sort()
+    #print(lst)
+    return lst[0][1]
+
+import unittest
+
+class TestMflp(unittest.TestCase):
+    def test1(self):
+        for inpt, otpt in {
+                'abcdbca': 'bc',
+                'cdeabeabfcdfabgcd': 'ab',
+                }.items():
+            self.assertEqual(mflp(inpt),otpt,f"error: most frequent letter pair of '{inpt}' must be '{otpt}'") 
+
+unittest.main()