Find First and Last Position of Element in Sorted Array

Problem 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.

Example 1:

Input: nums = [5,7,7,8,8,10], target = 8
Output: [3,4]

Example 2:

Input: nums = [5,7,7,8,8,10], target = 6
Output: [-1,-1]

Example 3:

Input: nums = [], target = 0
Output: [-1,-1]

Constraints:

• 0 <= nums.length <= 105
• -109 <= nums[i] <= 109
• nums is a non-decreasing array.
• -109 <= target <= 109

Solution (JavaScript)

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var searchRange = function(nums, target) {
    let firstleft = 0, lastleft = 0;
    let firstright = nums.length - 1, lastright = nums.length - 1;
    let first = -1, last = -1;
    let mid;
    
    while (firstleft <= firstright) {
        mid = Math.floor((firstleft + firstright) / 2);

        if (nums[mid] === target && ( nums[mid-1] < nums[mid] || mid === 0)) {
            first = mid;
            break;
        } else if (nums[mid] < target) {
            firstleft = mid + 1;
        } else {
            firstright = mid - 1;
        }
    }
    
    while (lastleft <= lastright) {
        mid = Math.floor((lastleft + lastright) / 2);

        if (nums[mid] === target && ( nums[mid+1] > nums[mid] || mid === nums.length-1)) {
            last = mid;
            break;
        } else if (nums[mid] > target) {
            lastright = mid - 1;
        } else {
            lastleft = mid + 1;
        }
    }
    
    return [first, last];
};