34. Find First and Last Position of Element in Sorted Array

• categories: Code, Interview Question, leetcode, Medium
• source: https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array
• topics: Binary Search, Array

Description

Given an array of integers nums sorted in non-decreasing order, find the starting and ending position of a given target value.

If target is not found in the array, return [-1, -1].

You must write an algorithm with O(log n) runtime complexity.

Idea

• Find left and right boundaries separately using binary search
• Check for edge cases in the end

Code

class Solution:
    def searchRange(self, nums: List[int], target: int) -> List[int]:
        if not nums:
            return [-1, -1]
 
        def find_leftmost(nums, target):
            left, right = 0, len(nums) - 1
            while left <= right:
                mid = (left + right) // 2
                if nums[mid] < target:
                    left = mid + 1
                else:
                    right = mid - 1
            return left
 
        def find_rightmost(nums, target):
            left, right = 0, len(nums) - 1
            while left <= right:
                mid = (left + right) // 2
                if nums[mid] > target:
                    right = mid - 1
                else:
                    left = mid + 1
            return right
 
        left_idx = find_leftmost(nums, target)
        right_idx = find_rightmost(nums, target)
 
        # Check if the target is actually in the range
        if left_idx <= right_idx and left_idx < len(nums) and nums[left_idx] == target:
            return [left_idx, right_idx]
        else:
            return [-1, -1]