# Rotating Leaders: Perl Challenge 78

### TASK #1 › Leader Element

Submitted by: Mohammad S Anwar

You are given an array @A containing distinct integers.Write a script to find all leader elements in the array @A. Print (0) if none found.

An element is leader if it is greater than all the elements to its right side.

So, we can go through every subarray we want with `while (array)`

and `shift array`

;

```
9 10 7 5 6 1
10 7 5 6 1
7 5 6 1
5 6 1
6 1
1
```

And so we’re comparing the first element of the array, `$arr[0]`

, and the largest value of the array, `max @arr`

from List::Util, one of the great gifts Perl gives us.

This makes it very easy to handle this.

#### Code

```
#!/usr/bin/env perl
use strict;
use warnings;
use feature qw{ say signatures state };
no warnings qw{ experimental };
use List::Util qw{ max };
# You are given an array @A containing distinct integers.
# Write a script to find all leader elements in the array @A.
# Print (0) if none found.
# Input: @A = (9, 10, 7, 5, 6, 1)
# Output: (10, 7, 6, 1)
my @A;
@A = ( 9, 10, 7, 5, 6, 1 );
my @o1 = leader_element(@A);
say join ', ', @A;
say join ', ', @o1;
say '';
# Input: @A = (3, 4, 5)
# Output: (5)
@A = ( 3, 4, 5 );
my @o2 = leader_element(@A);
say join ', ', @A;
say join ', ', @o2;
say '';
sub leader_element ( @arr ) {
my @output;
while (@arr) {
my $max = max @arr;
push @output, $arr[0] if $max == $arr[0];
shift @arr;
}
push @output, 0 unless scalar @output;
return wantarray ? @output : \@output;
}
```

And, knowing both the input and the expected output, I’m thinking this is exactly the kind of code I could write using Test modules to get my head more fully into that.

### TASK #2 › Left Rotation

Submitted by: Mohammad S Anwar You are given array @A containing positive numbers and @B containing one or more indices from the array @A.

Write a script to left rotate @A so that the number at the first index of @B becomes the first element in the array. Similary, left rotate @A again so that the number at the second index of @B becomes the first element in the array.

It took the examples for me to *get* this. First-pass is easy to get. We have the array of example 1, `[10, 20, 30, 40, 50]`

, and one of the indexes, `3`

.

```
0 1 2 3 4
10 20 30 40 50
```

So, we have the index of 3, so we want `40`

to be the first in the array, with all the ones before thrown to the end.

```
40 50 10 20 30
```

And then we’re given a new index, `4`

, which makes the rotation on the last element of the array. So, are we to go forward with the modified array, in which case the last element becoming first is `30`

? Or are we to go back to the original, where it’s `50`

?

The example output says it’s `50`

, so I go forward with that.

So, `for (1 .. $i) { push @arr, shift @arr }`

is enough to move the array around.

#### Code

```
#!/usr/bin/env perl
use strict;
use warnings;
use feature qw{ say signatures state };
no warnings qw{ experimental };
# You are given array @A containing positive numbers
# and @B containing one or more indices from the array @A.
# Write a script to left rotate @A so that the number
# at the first index of @B becomes the first element
# in the array. Similary, left rotate @A again so that
# the number at the second index of @B becomes the first
# element in the array.
# and this is where you look into the examples to tell,
# because by description, I would've thought that you
# work again on the modified array, not starting over
my $o1 = left_rotation( [ 10, 20, 30, 40, 50 ], [ 3, 4 ] );
my $o2 = left_rotation( [ 7, 4, 2, 6, 3 ], [ 1, 3, 4 ] );
for my $o ( $o1, $o2 ) {
for my $r ( $o->@* ) {
print '[';
print join ' ', $r->@*;
say ']';
}
say '';
}
# Example 1
# [40 50 10 20 30]
# [50 10 20 30 40]
# Example 2
# [4 2 6 3 7]
# [6 3 7 4 2]
# [3 7 4 2 6]
sub left_rotation ( $nums, $indices ) {
my @output;
for my $i ( $indices->@* ) {
my @new = $nums->@*;
for ( 1 .. $i ) {
push @new, shift @new;
}
push @output, [@new];
}
push @output, 0 unless scalar @output;
return wantarray ? @output : \@output;
}
```