Solution: First Non-Repeating Integer in a List—Hashing

Statement

Given a list of integers, nums, find the first non-repeating integer in the list.

Constraints:

• $1 \leq$ nums.length $\leq 10^3$

• $-5000 \leq$ nums[i] $\leq 5000$

Solution 1: Using a dictionary to keep count of repetitions

We track the frequency of each element in the list using a dictionary, which stores key-value pairs. The keys represent the elements of the list, 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 list of elements.

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

   2. If the element is not in counts, add it to the dictionary with a count of $1$.

3. Iterate through the list again.

   1. Check the count of each element in counts.

   2. Return the first element with a count of $1$.

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