1752. Check if Array Is Sorted and Rotated #

Problem #

Given an array nums, return true if the array was originally sorted in non-decreasing order, then rotated some number of positions (including zero). Otherwise, return false.

There may be duplicates in the original array.

Note: An array A rotated by x positions results in an array B of the same length such that A[i] == B[(i+x) % A.length], where % is the modulo operation.

Example 1:

Input: nums = [3,4,5,1,2]
Output: true
Explanation: [1,2,3,4,5] is the original sorted array.
You can rotate the array by x = 3 positions to begin on the the element of value 3: [3,4,5,1,2].

Example 2:

Input: nums = [2,1,3,4]
Output: false
Explanation: There is no sorted array once rotated that can make nums.

Example 3:

Input: nums = [1,2,3]
Output: true
Explanation: [1,2,3] is the original sorted array.
You can rotate the array by x = 0 positions (i.e. no rotation) to make nums.

Example 4:

Input: nums = [1,1,1]
Output: true
Explanation: [1,1,1] is the original sorted array.
You can rotate any number of positions to make nums.

Example 5:

Input: nums = [2,1]
Output: true
Explanation: [1,2] is the original sorted array.
You can rotate the array by x = 5 positions to begin on the element of value 2: [2,1].

Constraints:

1 <= nums.length <= 100
1 <= nums[i] <= 100

Problem Summary #

Given an array nums. In the source array of nums, all elements are the same as in nums, but arranged in non-decreasing order. If nums can be obtained by rotating the source array by several positions (including 0 positions), return true; otherwise, return false. There may be duplicates in the source array.

Solution Approach #

Easy problem. Scan the array once from the beginning and find pairs of adjacent elements that are decreasing. If there is only 1 decreasing pair, it may have been obtained by rotation; if there is more than 1, return false. The statement also mentions that there may be multiple duplicate elements. For this case, you also need to check nums[0] and nums[len(nums)-1]. If they are not the same element, nums[0] < nums[len(nums)-1], and there is also a decreasing pair in the middle of the array, then this is also false. After handling the above 2 cases, the problem is solved.

Code #

package leetcode

func check(nums []int) bool {
	count := 0
	for i := 0; i < len(nums)-1; i++ {
		if nums[i] > nums[i+1] {
			count++
			if count > 1 || nums[0] < nums[len(nums)-1] {
				return false
			}
		}
	}
	return true
}