What is the best algo for this array minimization problem?

Question

I have an array say 'A' of size n having some numbers {a1, a2, …, an} not necessarily distinct. I have to create another array B = {b1, b2, …, bn} which are distinct such that the value of sum|ai - bi| over all i's{i =1 to i =n} is minimized.

Basically I want to minimize sum of |ai - bi| over all i

What is the best algo for this?

I tried a greedy approach:

pseudocode:

for i = 0 to n-1{
 if(a[i] not in b){
  b[i] = a[i];}
 else{
  cnt = 1
  assigned = false
 do{
  if(a[i]-cnt not in b){
   b[i] = a[i]-cnt;
   assigned = true}
  elif(a[i]+cnt not in b){
   b[i] = a[i]+cnt;
   assigned = true}
  else
   cnt++
 }while(assigned==false)
}//else
}//for loop

NOte: 'n' is an input variable. the goal is to minimize sum of |ai - bi| over all i

Welcome to SO. Firstly, please tag a language. Secondly, please show us what you've tried. — Mitya, Nov 23 '20 at 10:59
No, don't tag a language. Tag [language-agnostic] if you must. — David Eisenstat, Nov 23 '20 at 11:46
What is the expected output? The `b[i]` or the minimum sum? And what is the maximum size `n` of the problem? — Damien, Nov 23 '20 at 12:11
@Damien 'n' is an input variable., a is an array of length 'n' which is also an input the goal is to create an array b(of len n) so that it minimizes sum of |ai - bi| over all i — Shirish Srivastava, Nov 23 '20 at 12:34

score 0 · Accepted Answer · answered Nov 23 '20 at 13:34

I came up with a O(NlogN) solution. Its based on sorting the input-sequence and greedily expanding the available numbers around it.

Code implementation in Python:


def get_closest_distinct_tuple(X: list):
    X = sorted(X, reverse=True)
    hmap = {}
    used_set = set()

    Y = []

    for x in X:
        if x not in used_set:
            Y.append(x)
            hmap[x] = 1
            used_set.add(x)
        else:
            Y.append(x + hmap[x])
            used_set.add(x + hmap[x])
            hmap[x] = 1 - hmap[x] if hmap[x] < 0 else -hmap[x]

    dist = sum([abs(X[i]-Y[i]) for i in range(len(X))])
    return dist, Y


print(get_closest_distinct_tuple([20, 1, 1, 1, 1, 1, 1]))

Output:

Dist: 9
Y = [20, 1, 2, 0, 3, -1, 4]

I couldnt really find a way to prove that this is the most optimal solution out there.

What is the best algo for this array minimization problem?

1 Answers1

Code implementation in Python: