# RogerBW's Blog

 Perl Weekly Challenge 56: Diff-K and path sum 29 April 2020 I’ve been doing the Perl Weekly Challenges. The latest involved searching arrays and summing tree paths. You are given an array @N of positive integers (sorted) and another non negative integer k. Write a script to find if there exists 2 indices i and j such that A[i] - A[j] = k and i != j. It should print the pairs of indices, if any such pairs exist. Example: ``````@N = (2, 7, 9) \$k = 2 `````` Output : 2,1 This feels like someone's computer science homework. But it's simple enough in O(N²); if the numbers were unique I could optimise by sticking them into hash keys and seeing whether the hash entry for (k-N[i]) existed, but that's not a guarantee given here. ``````my @N = (2, 7, 9); my \$k = 2; my @out; foreach my \$j (0..\$#N-1) { foreach my \$i (\$j+1..\$#N) { if (\$N[\$i]-\$N[\$j] == \$k) { push @out,[\$i,\$j]; } } } foreach (@out) { print "\$_->[0], \$_->[1]\n"; } `````` Perl6 lets me get rid of the references but is otherwise the same. You are given a binary tree and a sum, write a script to find if the tree has a path such that adding up all the values along the path equals the given sum. Only complete paths (from root to leaf node) may be considered for a sum. Example Given the below binary tree and sum = 22, `````` 5 / \ 4 8 / / \ 11 13 9 / \ \ 7 2 1 `````` For the given binary tree, the partial path sum 5 → 8 → 9 = 22 is not valid. The script should return the path 5 → 4 → 11 → 2 whose sum is 22. The interesting part here is not the actual path walking but the representation of the tree. Since I know I only want to walk it downwards, I only care about the descendants of any particular node. My input representation uses leading . to indicate depth. ``````use List::Util qw(sum); my @tree=qw(5 .4 ..11 ...7 ...2 .8 ..13 ..9 ...1); # 0 1 2 3 4 5 6 7 8 my \$target=22; my %tree; { my @parent; foreach my \$index (0..\$#tree) { \$tree[\$index] =~ /^(\.*)(\d+)/; my (\$depth,\$val)=(length(\$1),\$2); \$tree{\$index}{value}=\$val; if (\$depth>0) { push @{\$tree{\$parent[\$depth-1]}{children}},\$index; } \$parent[\$depth]=\$index; } } `````` Now that we have the tree, a standard breadth-first search implemented as a ring buffer will solve the nominal problem. (I could optimise by storing the sum of the path so far in the path entry, but scaling up isn't one of the requirements of the problem.) ``````my @paths=([0]); while (@paths) { my \$p=shift @paths; if (exists \$tree{\$p->[-1]}{children}) { foreach my \$x (@{\$tree{\$p->[-1]}{children}}) { push @paths,[@{\$p},\$x]; } } else { if (sum(map{\$tree{\$_}{value}} @{\$p}) == \$target) { print join(' -> ',map{\$tree{\$_}{value}} @{\$p}),"\n"; } } } `````` Comments on this post are now closed. If you have particular grounds for adding a late comment, comment on a more recent post quoting the URL of this one. Search 10 per page 20 per page 50 per page 100 per page Archive Tags 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s 3d printing action aeronautics aikakirja anecdote animation anime army astronomy audio audio tech aviation base commerce battletech beer boardgaming book of the week bookmonth chain of command children chris chronicle church of no redeeming virtues cold war comedy computing contemporary cornish smuggler cosmic encounter coup covid-19 cycling dead of winter doctor who documentary drama driving drone ecchi economics espionage essen 2015 essen 2016 essen 2017 essen 2018 essen 2019 existential risk falklands war fandom fanfic fantasy feminism film firefly first world war flash point flight simulation food garmin drive gazebo genesys geocaching geodata gin gkp gurps gurps 101 gus harpoon historical history horror hugo 2014 hugo 2015 hugo 2016 hugo 2017 hugo 2018 hugo 2019 hugo 2020 hugo-nebula reread in brief avoid instrumented life javascript julian simpson julie enfield kickstarter kotlin learn to play leaving earth linux liquor lovecraftiana lua mecha men with beards museum music mystery naval noir non-fiction one for the brow opera parody paul temple perl perl weekly challenge photography podcast politics postscript powers prediction privacy project woolsack pyracantha python quantum rail raku ranting raspberry pi reading reading boardgames social real life restaurant reviews romance rpg a day rpgs ruby rust science fiction scythe second world war security shipwreck simutrans smartphone south atlantic war squaddies stationery steampunk stuarts suburbia superheroes suspense television the resistance the weekly challenge thirsty meeples thriller tin soldier torg toys trailers travel type 26 type 31 type 45 vietnam war war wargaming weather wives and sweethearts writing about writing x-wing young adult Special All book reviews, All film reviews
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