Target occurs multiple times
Input
nums = [2, 4, 4, 4, 6], target = 4
Output
[1, 3]
The target 4 first appears at index 1 and last at index 3.
Full lesson preview
Search for a range of a target in a sorted array
Find the starting and ending positions of a target value in a sorted array using binary search.
Python practice15 minSearching & SortingIntermediateLast updated March 25, 2026
Problem statement
Given a sorted list of integers nums and an integer target, return a list [first_index, last_index] representing the first and last positions of target in nums. If target is not found, return [-1, -1]. Achieve O(log n) time using binary search (no full linear scan).
Task
Implement an efficient O(log n) search to find the first and last occurrence indices of a target in a sorted list.
Examples
Input format
Function call: search_range(nums, target) where nums is a sorted list of integers and target is an integer.
Output format
Return a list [first_index, last_index] (both integers). If not found return [-1, -1].
Constraints
- 0 <= len(nums) <= 10^5
- nums is non-decreasing (sorted)
- Use binary search for O(log n) time
Samples
Sample 1
Input
search_range([1,2,3,3,3,4], 3)
Output
[2, 4]
3 appears from index 2 through 4.