[LeetCode 34] Find First and Last Position of Element in Sorted Array

Problem

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

Thoughts

Modified binary search

Typescript

function searchRange(nums: number[], target: number): number[] {
    const binSearch = (nums, target, leftBias) => {
        let l = 0, r = nums.length - 1;

        let res = -1;
        while(l <= r) {
            let m = Math.floor((l + r)/2)

            
            if(target < nums[m]){
                r = m - 1
            } else if(target > nums[m]){
                l = m + 1
            } else {
                res = m
                // trying to find left most index
                if(leftBias) {
                    r = m - 1
                } else {
                    l = m + 1
                }
            }
        }
        
        return res;
    }
    
    return [binSearch(nums, target, true), binSearch(nums, target, false)]
};

Reference

https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array/

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