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#!/usr/bin/env perl
use strict;
use warnings;
use experimental qw{ say postderef signatures state };
my @examples = (
[ 1, 2, 2, 3 ],
[ 1, 3, 2 ],
[ 6, 5, 5, 4 ],
[ map { 7 } 1 .. 6 ],
);
for my $e (@examples) {
my $list = join ',', $e->@*;
my $out = monotonic( $e->@* );
say <<"END";
Input: \@array = ($list)
Output: $out
END
}
# Monotone increasing: for i <= j , nums[i] <= nums[j]
# Monotone decreasing: for i <= j , nums[i] >= nums[j]
# first pass I wrote, I made it so that i was less than j by 1,
# not all indexes less than j. (In my code, I reverse i and j
# because I always use i for index of the main loop)
# So, we test to see if there's been both increasing and decreasing.
# The "spaceship" operator is most often used in sorting numeric
# values, which will give us a 1 if the first value is larger
# than the second, -1 if the opposite, and 0 if they're the same.
# We watch for increasing and decreasing, and if we get values
# for both, we return 0. At the end, if we don't find non-monotonic
# values, we return 1.
sub monotonic (@array) {
my $increasing = 0;
my $decreasing = 0;
for my $i ( 1 .. -1 + scalar @array ) {
for my $j ( 0 .. -1 + $i ) {
my $k = $array[$j] <=> $array[$i];
$decreasing++ if $k == -1;
$increasing++ if $k == 1;
return 0 if $decreasing && $increasing;
}
}
return 1;
}
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