# LeetCode 153. Find Minimum in Rotated Sorted Array

## The Problem

Link to original problem on LeetCode.

Suppose an array of length n sorted in ascending order is rotated between 1 and n times. For example, the array nums = [0,1,2,4,5,6,7] might become:

• [4,5,6,7,0,1,2] if it was rotated 4 times.
• [0,1,2,4,5,6,7] if it was rotated 7 times.

Notice that rotating an array [a, a, a, ..., a[n-1]] 1 time results in the array [a[n-1], a, a, a, ..., a[n-2]].

Given the sorted rotated array nums of unique elements, return the minimum element of this array.

You must write an algorithm that runs in $O(\log n)$ time.

Examples

Example 1:

Input: nums = [3,4,5,1,2]
Output: 1
Explanation: The original array was [1,2,3,4,5] rotated 3 times.


Example 2:

Input: nums = [4,5,6,7,0,1,2]
Output: 0
Explanation: The original array was [0,1,2,4,5,6,7] and it was rotated 4 times.


Example 3:

Input: nums = [11,13,15,17]
Output: 11
Explanation: The original array was [11,13,15,17] and it was rotated 4 times.

Constraints
• n == nums.length
• 1 <= n <= 5000
• -5000 <= nums[i] <= 5000
• All the integers of nums are unique.
• nums is sorted and rotated between 1 and n times.

## My Solution

The naïve solution would be return Math.min(...nums). This is $O(n)$ time, because it has to check every value in the nums array. We can instead use a modified binary search to get down to $O(\log n)$ time. LeetCode has a more detailed explanation with the problem on their site.

This first example uses recursion.

const findMin = (nums) => {    // If there's one value, return it.    // If the first value is less than the last,    // the array is sorted. Return the first value.    if (nums.length === 1 || nums < nums[nums.length - 1]) {        return nums;    }    // Find the mid-point of the array.    const mid = Math.floor(nums.length / 2);    // If the mid-point is greater than the value to it's right,    // then that right value is the lowest. Return it.    if (nums[mid] > nums[mid + 1]) {        return nums[mid + 1];    }    // Likewise, if the mid-point is less than the value to the    // left, then the mid-point is the lowest value. Return it.    if (nums[mid - 1] > nums[mid]) {        return nums[mid];    }    // If none of the previous conditions work, we'll    // recursively call the function on a portion of the array.    // If the first value is less than the mid-point, then the    // lowest value must still be on the right-hand side.    // Call the function again with the subarray of everything    // to the right of the mid-point.    // Otherwise, it's in the left-hand side.    if (nums < nums[mid]) {        return findMin(nums.slice(mid + 1));    } else {        return findMin(nums.slice(0, mid));    }};

This problem can also be solved without recursively calling the function. Instead, we use a while loop. This solution is, in my aesthetic opinion, superior to the first.

const findMin = (nums) => {  // Set left and right to the start and end indices of nums.  // The result will eventually be stored in the left variable.  let left = 0;  let right = nums.length - 1;  // Since an array with length 1 won't satisfy this condition,  // it skips straight to returning nums[left], which is nums  while (left < right) {    const mid = Math.floor((left + right) / 2);    if (nums[mid] > nums[right]) {      // If the mid-point is greater than the value at index      // right, then we know the lowest value is on the right      // side of the array. Set the left-most index to the      // mid-point.      left = mid + 1;    } else {      // Otherise, it must be on the left side. Set the new      // right-most index to the mid-point.      right = mid;    }  }  // Eventually, nothing will remain but the lowest value  // at the left-most index.  return nums[left];};