Find First and Last Position of Element in Sorted Array

Medium · binary search solution for Find First and Last Position of Element in Sorted Array. Python, Go, and a step-by-step animation.

Problem

Read the prompt, flip through the examples, then steal the idea.

LeetCode 34. Find First and Last Position of Element in Sorted Array. The copyrighted prompt is on LeetCode. This page is a working binary search solution for this problem: editable input, Python, Go, and a step-by-step animation.

Constraints

—Player inputs are capped so the animation stays readable.
—Match LeetCode's own constraints when you submit there.

Examples

Input

nums =: [-1, 0, 3, 5, 9, 12]
target =: 9

Output

9 sits at index 4. Each step throws away half the range.

Solution

Python and Go. The walkthrough below runs this same code.

Approach 1Optimal▶ Animated

Find First and Last Position of Element in Sorted Array — binary search

Halve the sorted range until the target is found or the range collapses. Built for LeetCode 34. Find First and Last Position of Element in Sorted Array.

Because the slice is sorted, comparing nums[mid] to target tells you which half can be thrown away. That is the whole algorithm.

Algorithm

While lo <= hi, pick mid.
Equal → return mid. Too small → search right. Too big → search left.
Empty range → not found (-1).

Time: O(log n)
Space: O(1)

Solution.py

1class Solution:2    def findFirstAndLastPositionOfElementInSortedArray(self, nums: list[int], target: int) -> int:3        lo, hi = 0, len(nums) - 14        while lo <= hi:5            mid = (lo + hi) // 26            if nums[mid] == target:7                return mid8            if nums[mid] < target:9                lo = mid + 110            else:11                hi = mid - 112        return -1

Watch this run

Interactive walkthrough

Playing “Find First and Last Position of Element in Sorted Array — binary search”. Edit the input, hit Run, then scrub.

Approach: Find First and Last Position of Element in Sorted Array — binary search

Loading animation…