The first solo ticket

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The brewery printer recorded a sequence of numbered tickets. Some numbers were printed more than once after customers changed their orders. Find the earliest ticket whose number occurs exactly once in the entire sequence. Return its zero-based index, or -1 if there is no such ticket. A ticket that looks unique at first may be repeated later.

Example 1

INPUT[12,7,12,9,7,4]OUTPUT3

9 and 4 appear once. The earlier of them is 9 at index 3.

Example 2

INPUT[0,8,8]OUTPUT0

Ticket 0 appears once and its index is 0; neither zero is a missing value.

Keep in mind

0 <= tickets.length <= 20,000
Every ticket number is an integer from 0 to 1,000,000.

/** @param {number[]} tickets
 *  @returns {number} */
function solve(tickets) {
  // Find the first number that occurs exactly once.
}

/** @param {number[]} tickets
 *  @returns {number} */
function solve(tickets) {
  // Find the first number that occurs exactly once.
}

INPUT[12,7,12,9,7,4]

EXPECTED OUTPUT3

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AFTER THE SOLVE

How the solution works.

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The first solo ticket

Example 1

Example 2

Keep in mind

How the solution works.

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