You are given an array of integers.

Write a script to return the number of reverse pairs in the given array.

A reverse pair is a pair (i, j) where: a) 0 <= i < j < nums.length and b) nums[i] > 2 * nums[j].

This is another case where the best way is iterative using nested arrays. I’m always iffy as to whether this gets O(nlogn) or not. But once we have the correct arrays, so we can’t compare a number with itself, we just do some simple math and comparisons, and we add the number when it’s correct.

#### Show Me The Code

``````#!/usr/bin/env perl

use strict;
use warnings;
use experimental qw{ say postderef signatures state };

use List::Compare;

my @examples = (

[ 1, 3, 2, 3, 1 ],
[ 2, 4, 3, 5, 1 ],
);
for my \$e (@examples) {
my \$output = reverse_pairs( \$e->@* );
my \$input  = join ', ', \$e->@*;
say <<~"END";
Input:  \@input = (\$input)
Output: \$output
END
}

sub reverse_pairs (@input) {
my \$output = 0;
for my \$i ( 0 .. -1 + scalar @input ) {
my \$ii = \$input[\$i];
for my \$j ( \$i + 1 .. -1 + scalar @input ) {
my \$jj = \$input[\$j];
\$output++ if \$ii > \$jj * 2;
}
}
return \$output;
}
``````
``````\$ ./ch-1.pl
Input:  @input = (1, 3, 2, 3, 1)
Output: 2

Input:  @input = (2, 4, 3, 5, 1)
Output: 3
``````

You are given an array of positive integers (>=1).

Write a script to return the sum of floor(nums[i] / nums[j]) where 0 <= i,j < nums.length. The floor() function returns the integer part of the division.

Again with the nested loops, and because it’s whole-array vs whole-array, I think this is definitely O(n2), which means handling larger arrays would get suckier quickly, but the floor is the integer part of division, so `int( \$num / \$denom )` will do it. We could use `sum0` from List::Util (always a useful go-to) and replace the inner loop with `sum0 map { int( \$ii / \$_ ) } @input`, but I don’t like going functional unless I can go fully functional, and with two arrays, it kinda gets ugly.

(I never liked Perl Golf-style development, and try, even in my toy code, to write things I’ll understand next time I read them.)

#### Show Me The Code

``````#!/usr/bin/env perl

use strict;
use warnings;
use experimental qw{ say postderef signatures state };

my @examples = (

[ 2, 5, 9 ],
[ 7, 7, 7, 7, 7, 7, 7 ],
);

for my \$e (@examples) {
my \$input  = join ', ', \$e->@*;
my \$output = floor_sum( \$e->@* );

say <<~"END";
Input:  \\$input = (\$input)
Output:          \$output
END
}

sub floor_sum (@input) {
my \$output = 0;
for my \$i ( 0 .. -1 + scalar @input ) {
my \$ii = \$input[\$i];
for my \$j ( 0 .. -1 + scalar @input ) {
my \$jj    = \$input[\$j];
my \$floor = int( \$ii / \$jj );
\$output += \$floor;
}
}
return \$output;
}
``````
``````\$ ./ch-2.pl
Input:  \$input = (2, 5, 9)
Output:          10

Input:  \$input = (7, 7, 7, 7, 7, 7, 7)
Output:          49
``````