awelgorithms

Group indices per value

Updated

Value → list of indices where it appears. This is the grouping you want when the problem asks every index of 1, not just first or last.

What to focus on

Build in one left-to-right pass so lists are sorted. Empty lists are absent keys, not stored.

Worked examples

repeats

a = [4, 1, 9, 1]
  1. 4→[0]
  2. 1→[1,3]
  3. 9→[2]

Result: {4:[0], 1:[1,3], 9:[2]}

all same

a = [7, 7, 7]
  1. 7→[0,1,2]

Result: {7:[0,1,2]}

empty

a = []
  1. no indices

Result: {}

Try it

Write indexLists(a). a is an array of integers. Return value → list of indices, built left to right so lists are increasing. a = [4, 1, 9, 1] → {4:[0], 1:[1,3], 9:[2]}. a = [7, 7, 7] → {7:[0,1,2]}. a = [] → {}.

a is a list of integers and may be empty.

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