Find First and Last Occurrences in a Sorted Array

Problem

Given a sorted array arr that may contain duplicates, find the first and last occurrence of a target element x.
If x is not present, return [-1, -1].

Examples

Input
arr = [1, 3, 5, 5, 5, 5, 67, 123, 125], x = 5

Output
[2, 5]

Explanation: First occurrence of 5 is at index 2, last at index 5.

Input

arr = [1, 3, 5, 5, 5, 5, 7, 123, 125], x = 7

Output

[6, 6]

Explanation: First and last occurrence of 7 is at index 6.

Input
arr = [1, 2, 3], x = 4

Output
[-1, -1]

Explanation: 4 is not present.

Approach

Use binary search to find the first occurrence of x.
Use binary search again to find the last occurrence.
If x is not found in either step, return [-1, -1].
This is efficient because the array is sorted, giving O(log n) time complexity.

Python Code
class Solution:
def find(self, arr, x):
n = len(arr)
first = -1
last = -1

    # Find first occurrence
    low = 0
    high = n - 1
    while low <= high:
        mid = (low + high) // 2
        if arr[mid] == x:
            first = mid
            high = mid - 1
        elif arr[mid] < x:
            low = mid + 1
        else:
            high = mid - 1

    # Find last occurrence
    low = 0
    high = n - 1
    while low <= high:
        mid = (low + high) // 2
        if arr[mid] == x:
            last = mid
            low = mid + 1
        elif arr[mid] < x:
            low = mid + 1
        else:
            high = mid - 1

    return [first, last]

Output
[2, 5]