Find First Entry in List with Duplicates

In this lesson, we will be writing a function that takes an array of sorted integers and a key and returns the index of the first occurrence of that key from the array.

For example, for the array:

[-14, -10, 2, 108, 108, 243, 285, 285, 285, 401]

with

target = 108

the algorithm would return 3, as the first occurrence of 108 in the above array is located at index 3.

The most naive approach to solving this problem is to loop through each element in the array. If you stumble upon the target element, it will be the first occurrence because the array is sorted. Otherwise, if the number does not exist in the array, we return None to indicate that the number is not present in a list.