Find the First Array Element Smaller Than Both of Its Neighbors

Given an array of integers, find the first element, scanning from left to right, whose two adjacent neighbors are both strictly larger than it.

Function Signature

def first_dip(nums: list[int]) -> int:

Rules

• The element at index i has two adjacent neighbors only when 0 < i < len(nums) - 1. The first and last elements never qualify.
• The element at index i qualifies when nums[i - 1] > nums[i] and nums[i + 1] > nums[i]. A neighbor equal to the element does not count as larger.
• Return the value of the qualifying element with the smallest index.
• Return -1 if no element qualifies, including when the array has fewer than 3 elements.

Constraints

• 1 <= len(nums) <= 10^5
• 0 <= nums[i] <= 10^9, so the result -1 can never be confused with an element.

Examples

Example 1

Input: nums = [5, 3, 4, 1, 2]
Output: 3

The element 3 at index 1 has neighbors 5 and 4, both larger. The element 1 at index 3 also qualifies, but it comes later.

Example 2

Input: nums = [2, 2, 7, 6, 9, 0]
Output: 6

Index 1 does not qualify because its left neighbor 2 is equal, not larger. Index 2 holds 7, which is larger than both of its neighbors. Index 3 holds 6, whose neighbors 7 and 9 are both larger.

Example 3

Input: nums = [1, 2, 2, 3]
Output: -1

Index 1 has the smaller neighbor 1, and index 2 has the equal neighbor 2, so no element qualifies.