Leetcode•Aug 19, 2025
Find First and Last Position of Element in Sorted Array
Hazrat Ali
Leetcode
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]
Solution
/**
* @param {number[]} nums
* @param {number} target
* @return {number[]}
*/
const searchRange = (nums, target) => {
const lower = searchLowerBound(nums, target);
const upper = searchUpperBound(nums, target);
if (lower === -1) {
return [-1, -1];
}
return [lower, upper];
};
const searchLowerBound = (nums, target) => {
let lo = 0;
let hi = nums.length - 1;
while (lo <= hi) {
const mid = lo + Math.floor((hi - lo) / 2);
if (nums[mid] < target) {
lo = mid + 1;
} else if (nums[mid] > target) {
hi = mid - 1;
} else {
if (mid === 0 || nums[mid - 1] < target) {
return mid;
} else {
hi = mid - 1;
}
}
}
return -1;
};
const searchUpperBound = (nums, target) => {
let lo = 0;
let hi = nums.length - 1;
while (lo <= hi) {
const mid = lo + Math.floor((hi - lo) / 2);
if (nums[mid] < target) {
lo = mid + 1;
} else if (nums[mid] > target) {
hi = mid - 1;
} else {
if (mid === nums.length - 1 || nums[mid + 1] > target) {
return mid;
} else {
lo = mid + 1;
}
}
}
return -1;
};