Hash Tables

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 JavaScript DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.js 
const arr = [3, 5, 2, 5, 3, 8, 2];
const count = new Map();
for (const value of arr) {
  count.set(value, (count.get(value) ?? 0) + 1);
}
for (const value of arr) {
  if (count.get(value) === 1) {
    console.log(value);
    break;
  }
}

Complexity

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

Implementation notes

  • Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
  • The trace highlights the hash table state after each write.
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.