First Non-Repeating Value

Find the first input value whose final frequency is one.

Algorithm

Canonical input [3, 5, 2, 5, 3, 8, 2] prints 8. The replay uses the same input in every language, so this Ruby DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.rb

arr = [3, 5, 2, 5, 3, 8, 2]
count = Hash.new(0)
arr.each { |value| count[value] += 1 }
arr.each do |value|
  if count[value] == 1
    puts value
    break
  end
end

Complexity

Time: O(n) average
Space: O(k) for k distinct values

Implementation notes

arr is a Ruby array of integers, not a string, so both passes iterate values with arr.each.
count = Hash.new(0) creates a Ruby Hash whose missing keys read as 0.
The counting pass uses count[value] += 1; the default value makes the first increment write 1 without a separate key check.
After the first pass, the trace shows the frequency table as {3: 2, 5: 2, 2: 2, 8: 1}.
The second pass scans the original array order again, checking count[value] == 1.
Values 3, 5, 2, 5, and 3 are skipped because their counts are 2.
When the scan reaches value 8 at index 5, the source prints it with puts value and exits the loop with break.
The checked output is the single line 8; the source never prints the hash table itself.

two-pass lookup The first pass builds a frequency table. The second pass keeps the original order and stops at the first value with frequency one.