Blog for challenge 245

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+https://github.com/manwar/perlweeklychallenge-club/blob/master/challenge-245/jo-37/blog/Blog.md
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+# The Most Popular of Three
+
+## Task 1: Sort Language
+**Submitted by: Mohammad S Anwar**
+
+---
+You are given two array of languages and its popularity.
+
+Write a script to sort the language based on popularity.
+
+### Example 1
+```
+Input: @lang = ('perl', 'c', 'python')
+       @popularity = (2, 1, 3)
+Output: ('c', 'perl', 'python')
+```
+### Example 2
+```
+Input: @lang = ('c++', 'haskell', 'java')
+       @popularity = (1, 3, 2)
+Output: ('c++', 'java', 'haskell')
+```
+---
+### Solution
+Performing an index sort on the popularity and using the result as slice indices for the languages leads to a one-line solution:
+```
+sub sort_by_popularity ($lang, $p8y) {
+	$lang->@[sort {$p8y->[$a] <=> $p8y->[$b]} 0 .. $p8y->$#*];
+}
+
+```
+
+## Task 2: Largest of Three
+**Submitted by: Mohammad S Anwar**
+
+---
+You are given an array of integers >= 0.
+
+Write a script to return the largest number formed by concatenating some of the given integers in any order which is also multiple of 3. Return -1 if none found.
+
+### Example 1
+```
+Input: @digits = (8, 1, 9)
+Output: 981
+
+981 % 3 == 0
+
+```
+### Example 2
+```
+Input: @digits = (8, 6, 7, 1, 0)
+Output: 8760
+```
+### Example 3
+```
+Input: @digits = (1)
+Output: -1
+```
+---
+
+### Solution
+Some considerations:
+
+  * The largest number that can be formed from given digits has all the digits in descending order
+  * Any number without leading zeroes is larger than any other number that has less digits.
+  * A number is divisible by 3 if and only if the sum of its decimal digits is divisible by 3.
+
+To form the largest number divisible by 3 from the given digits, we primarily must use the maximum number of digits and secondarily the largest digits.
+Suppose we have `p = sum(d[i]) mod 3`.
+
+  * If `p = 0` we may use all the given digits
+  * If there are digits within the given set having `d = p mod 3`, we may drop the smallest of these digits to get the maximum number.
+  * Otherwise there must be two or more digits having `d = -p mod 3`. We may drop the smallest two of these to get the maximum number.
+
+If the remaining set of digits is not empty, the largest number is formed from the digits in descending order.
+Otherwise there is no solution.
+
+This results in the following implementation:
+```
+sub largest_of_three {
+	my (@digits, @ind, $mod) = sort @_;
+    for (0 .. $#digits) {
+        # The digit modulo 3
+        my $digit = $digits[$_] % 3;
+        # Store indices of digits that are not a multiple of 3
+        push $ind[$digit - 1]->@*, $_ if $digit;
+        # Update the digit sum
+        $mod += $digit;
+    }
+    $mod %= 3;
+    # Delete the fewest smallest digits to obtain a digit sum
+    # divisible by 3.
+    delete @digits[
+        !$mod-- ? () : $ind[$mod] ? $ind[$mod][0] : $ind[!$mod]->@[0, 1]
+    ];
+
+    # Build the maximum number from the remaining digits in descending
+    # order or fail.
+    @digits ? 0 + join '', reverse grep defined, @digits : -1;
+}
+
+```
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