The Take-back counting algorithm

Question

Kids who are learning how to count often know runs of numbers, but can't seem to put those runs together properly.

For example, they might say:

1,2,3,4,7,8,9,10

Sometimes kids will realize that they skipped some numbers, and go back:

1,2,3,4,7,8,5,6,7,8,9,10

This is clearly the superior pattern. We need to identify them.

To identify these lists:

1. We identify the minimum M and the maximum N of the list

2. We step through the list. If the current number is greater than or equal to any member of the list to its right, then we remove the current number.

3. Iff the remaining list contains all numbers from M to N, then we return a truthy value.

You can assume your input list will contain at least 1 element. You can assume that all integers will be non-negative.

Test cases:

Truthy:

0
10
0 0 0 
1 0 1
0 1 2 3 4 5 6 7 8 9 10
0 1 2 3 0 1 2 3
0 1 2 3 4 5 5
0 1 1 2 2 3
0 3 6 1 4 7 2 5 8 3 4 5 6 7 8
1 3 5 7 2 3 4 5 6 7
5 6 0 1 2 3 6 7 4 5 6 7
5 6 7 8
5 5 6 7 8
4 6 7 8 3 4 5 6 7 8

Falsy:

1 0
4 3 2 1
1 2 3 7 8 9
0 1 2 3 1 3
0 1 2 3 1 3 4
0 1 2 3 1 3 2 4
0 1 2 3 1 3 2 4 3
1 3 5 7 2 4 6 8
0 1 2 1 3 4 5 6
4 5 6 3 4 5

This is code-golf, so make your answers as short as possible!

Answer 1

05AB1E, 5 bytes

I am not 100% certain this works, but it passes all test cases and I couldn't find any situation where it fails.

Ú¥1QP

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Ú¥1QP   Main link. Argument a
Ú       Reverse uniquify a, keeps only last occurence of each element
 ¥      Get all deltas - all 1 if ascending list
  1Q    Compare all deltas to 1
    P   Product of all results

Answer 2

Jelly, 10 9 bytes

ṀrṂɓṚ«\Q⁼

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How it works

ṀrṂɓṚ«\Q⁼  Main link. Argument: A (array)

Ṁ          Yield the maximum of A.
  Ṃ        Yield the minimum of A.
 r         Yield R := [max(A), ... min(A).
   ɓ       Begin a new chain. Left argument: A. Right argument: R
    Ṛ      Reverse A.
     «\    Take the cumulative minimum.
       Q   Unique; deduplicate the results.
        ⁼  Compare the result with R.

Answer 3

Ruby, 59 54 41 bytes

->a{a.reverse|[]==[*a.max.downto(a.min)]}

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Answer 4

Python 2, 81 bytes

x=input();r=m=[]
for n in x[::-1]:r=[n][:n<m]+r;m=r[0]
print r==range(m,max(x)+1)

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Answer 5

PHP, 148 130 bytes

-18 bytes, thanks @Christoph

$a=explode(' ',$argn);$b=range(min($a),max($a));foreach($a as$i=>&$k)for(;++$i<count($a);)$k<$a[$i]?:$k=-1;echo!array_diff($b,$a);

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Answer 6

Java 8, 264 262 bytes

import java.util.*;l->{int m=Collections.max(l),n=Collections.min(l),i=0,q;for(;i<(q=l.size());i++)if(l.subList(i+1,q).size()>0&&l.get(i)>=Collections.min(l.subList(i+1,q)))l.remove(i--);for(i=0;n<=m;)if(i<l.size()&&l.get(i++)==n)n++;else return 0>1;return 1>0;}

Explanation:

Try it here.

import java.util.*;                 // Import for Collections

l->{                                // Method with integer-ArrayList parameter and boolean return-type
  int m=Collections.max(l),         //  Max of the list
      n=Collections.min(l),         //  Min of the list
      i=0,q;                        //  Two temp integers
  for(;i<(q=l.size());i++)          //  Loop (1) over the list
    if(l.subList(i+1,q).size()>0    //   If the sublist right of the current item is not empty
    &&l.get(i)>=Collections.min(l.subList(i+1,q))) 
                                    //   and if the current item is larger or equal to the lowest value of this sublist
      l.remove(i--);                //    Remove the current item from the main list
                                    //  End of loop (1) (implicit / single-line body)
  for(i=0;n<=m;)                    //  Loop (2) from min to max
    if(i<l.size()                   //   If the current item doesn't exceed the list's size
    &&l.get(i++)==n)                //   and the items are in order so far
      n++;                          //    Go to the next item
    else                            //   Else:
      return 0>1;//false            //    Return false
                                    //  End of loop (2) (implicit / single-line body)
  return 1>0;//true                 //  Return true
}                                   // End of method

Answer 7

R, 88 85 bytes

y=NULL;for(i in x<-scan())if(all(i<x[-(1:(F<-F+1))]))y=c(y,i);all(min(x):max(x)%in%y)

This can probably be golfed down further. Loops over the elements of x, checks if all upcoming values are bigger, and only then keeps that element. After the loop it creates a sequence from min(x) to max(x), and checks with %in% if all values are included in the pruned version of x.

Answer 8

JavaScript (ES6), 60 bytes

s=>(o={},s.reverse().every((n,i)=>!i|o[n+1]|o[n]&&(o[n]=1)))

Ungolfed:

s=>(
  o={},
  s.reverse().every((n,i)=>
    !i|o[n+1]|o[n]&&(o[n]=1)
  )
)

This is a simpler algorithm:

Iterate the array in reverse, and make sure each number (except the first) is one less than or equal to a number already seen.

Snippet:

f=

s=>(o={},s.reverse().every((n,i)=>!i|o[n+1]|o[n]&&(o[n]=1)))

console.log('Truthy');
console.log(f([0]))
console.log(f([10]))
console.log(f([0,0,0]))
console.log(f([1,0,1]))
console.log(f([0,1,2,3,4,5,6,7,8,9,10]))
console.log(f([0,1,2,3,0,1,2,3]))
console.log(f([0,1,2,3,4,5,5]))
console.log(f([0,1,1,2,2,3]))
console.log(f([0,3,6,1,4,7,2,5,8,3,4,5,6,7,8]))
console.log(f([1,3,5,7,2,3,4,5,6,7]))
console.log(f([5,6,0,1,2,3,6,7,4,5,6,7]))
console.log(f([5,6,7,8]))
console.log(f([5,5,6,7,8]))
console.log(f([4,6,7,8,3,4,5,6,7,8]))

console.log('Falsy');
console.log(f([1,0]))
console.log(f([4,3,2,1]))
console.log(f([1,2,3,7,8,9]))
console.log(f([0,1,2,3,1,3]))
console.log(f([0,1,2,3,1,3,4]))
console.log(f([0,1,2,3,1,3,2,4]))
console.log(f([0,1,2,3,1,3,2,4,3]))
console.log(f([1,3,5,7,2,4,6,8]))
console.log(f([0,1,2,1,3,4,5,6]))
console.log(f([4,5,6,3,4,5]))

Answer 9

Haskell, 62 bytes

g(a:b)=[a|all(a<)b]++g b
g a=a
f x=g x==[minimum x..maximum x]

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A direct implementation of the definition where g removes the elements if they are >= than the the elements to its right.

Answer 10

C#, 69 bytes

s=>s.Where((e,i)=>s.Skip(i+1).All(r=>e<r)).Count()==s.Max()-s.Min()+1

In short:
s = input (s)equence
take from s element where all items after this one (skip (I)ndex + 1 items), current value is higher
count these, and see if the amount left is equal to the amount expected ((max)imum value minus (min)imum) amount of numbers

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Answer 11

JavaScript (ES6), 82 73 72 70 bytes

Returns a boolean.

a=>a.filter((x,i)=>k-=a.every(y=>~i--<0|y>x,m=x>m?x:m),m=k=0)[0]+~m==k

How?

We iterate on each element x of the input array a, keeping track of the maximum encountered value m and the number -k of values that are not greater than or equal to any member to their right. By definition, valid values appear in strictly ascending order.

We use filter() rather than map(), so that all elements get filtered out until k turns negative. This allows us to isolate the first valid element, which is also guaranteed to be the minimum value of the array.

Finally, we test whether minimum - (maximum + 1) == -number_of_valid_elements:

a.filter(...)[0] + ~m == k

Test cases

let f =

a=>a.filter((x,i)=>k-=a.every(y=>~i--<0|y>x,m=x>m?x:m),m=k=0)[0]+~m==k

console.log('[Truthy]');
console.log(f([0]));
console.log(f([10]));
console.log(f([0,0,0,]));
console.log(f([1,0,1]));
console.log(f([0,1,2,3,4,5,6,7,8,9,10]));
console.log(f([0,1,2,3,0,1,2,3]));
console.log(f([0,1,2,3,4,5,5]));
console.log(f([0,1,1,2,2,3]));
console.log(f([0,3,6,1,4,7,2,5,8,3,4,5,6,7,8]));
console.log(f([1,3,5,7,2,3,4,5,6,7]));
console.log(f([5,6,0,1,2,3,6,7,4,5,6,7]));
console.log(f([5,6,7,8]));
console.log(f([5,5,6,7,8]));
console.log(f([4,6,7,8,3,4,5,6,7,8]));

console.log('[Falsy]');
console.log(f([1,0]));
console.log(f([4,3,2,1]));
console.log(f([1,2,3,7,8,9]));
console.log(f([0,1,2,3,1,3]));
console.log(f([0,1,2,3,1,3,4]));
console.log(f([0,1,2,3,1,3,2,4]));
console.log(f([0,1,2,3,1,3,2,4,3]));
console.log(f([1,3,5,7,2,4,6,8]));
console.log(f([0,1,2,1,3,4,5,6]));
console.log(f([4,5,6,3,4,5]));