Solution: First Non-Repeating Integer in an Array—Hashing

Let’s solve the First Non-Repeating Integer in an Array—Hashing problem.

We'll cover the following


Given an array of integers, nums, find the first non-repeating integer in the array.


  • 11 \leq nums.length 103\leq 10^3

  • 5000-5000 \leq nums[i] 5000\leq 5000


We track the frequency of each element in the array using a dictionary, which stores key-value pairs. The keys represent the elements of the array, and the values indicate how many times each element occurs.

  1. Initialize an empty dictionary, counts, to store the counts of each element.

  2. Iterate through the array of elements.

    1. If the element is already a key in counts, increment its value by 11 to represent its count.

    2. If the element is not in counts, add it to the dictionary with a count of 11.

  3. Iterate through the array again.

    1. Check the count of each element in counts.

    2. Return the first element with a count of 11.

Let’s look at the illustration below to better understand the solution:

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