- Added template for Challenge - 070.

1 files changed, 40 insertions, 0 deletions

diff --git a/challenge-070/sgreen/README.md b/challenge-070/sgreen/README.md
new file mode 100644
index 0000000000..a4a45d0687
--- /dev/null
+++ b/challenge-070/sgreen/README.md
@@ -0,0 +1,40 @@
+Solution by Simon Green
+# Perl Weekly Challenge 069
+
+## TASK #1 › Strobogrammatic Number
+
+This is a lot harder than it looks. My first thought is to just do a foreach loop between `$a` and `$b` and add it to the list if the number is strobogrammatic. However when I ran
+
+    perl -E '$b = 0; foreach my $a (0 ..  10**10) { $b++ } say $b;'
+
+on my PC, that took nearly 3 minutes. With $b being up to 10<sup>15</sup> this was not a feasible approach. Even if I only had all the numbers between 1 and 10<sup>15</sup> containing only the digits 0, 6, 8, 9 that was still going to be in the billions.
+
+So I took a different approach. I calculated the first half of all valid numbers, and then generated the second half. The logic is pretty simple. The first digit must be 6, 8 or 9 (as 0 isn't valid as a first digit). If the number of digits is odd, the middle number can only be 0 or 8.
+
+Taking this approach, I can generate all 49,150 strobogrammatic numbers between 1 and 10<sup>15</sup> in less than half a second.
+
+### Examples
+    » ./ch-1.pl 50 100
+    69 88 96
+
+    » ./ch-1.pl 50 900
+    69 88 96 609 689 808 888
+
+## TASK #2 › 0/1 String
+
+After the first task, these seemed a lot easier. But it wasn't entirely straight forward. I thought I had a bug in my code when it was spewing out digits on my screen. It turns out that is expected. The length of a number is twice the length of a previous number plus one. This meant the expected length was:
+
+    1st  1
+    2nd  3
+    3rd  7
+    4th  15
+    5th  31
+    10th 1,023
+    15th 32,767
+    20th 1,048,575
+    25th 33,554,431
+    30th 1,073,741,823
+
+No wonder the task was changed from 1,000 iterations to 30! It goes without saying I'm not posting an example here, as the final line is 1,073,741,823 characters long.
+
+Thankfully perl has a built in [reverse](https://perldoc.pl/functions/reverse) operator which does what you expect to do on a scalar (string). For the switch I used `tr/01/10/` which [translates](https://perldoc.pl/perlop#tr/SEARCHLIST/REPLACEMENTLIST/cdsr) the zeros to one and visa versa.
\ No newline at end of file