0162-find-peak-element.rb

# frozen_string_literal: true

# 162. Find Peak Element
# https://leetcode.com/problems/find-peak-element
# Medium

=begin
A peak element is an element that is strictly greater than its neighbors.

Given a 0-indexed integer array nums, find a peak element, and return its index. If the array contains multiple peaks, return the index to any of the peaks.

You may imagine that nums[-1] = nums[n] = -∞. In other words, an element is always considered to be strictly greater than a neighbor that is outside the array.

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

Example 1:
Input: nums = [1,2,3,1]
Output: 2
Explanation: 3 is a peak element and your function should return the index number 2.

Example 2:
Input: nums = [1,2,1,3,5,6,4]
Output: 5
Explanation: Your function can return either index number 1 where the peak element is 2, or index number 5 where the peak element is 6.

Constraints:
1 <= nums.length <= 1000
-231 <= nums[i] <= 231 - 1
nums[i] != nums[i + 1] for all valid i.
=end

# @param {Integer[]} nums
# @return {Integer}
def find_peak_element(nums)
  # n = nums.size - 1
  # (0..n).bsearch { |i| i == n || nums[i] > nums[i + 1] }
  (0...(nums << -10**15).length).bsearch { nums[_1] > nums[_1 + 1] }
end

# **************** #
#       TEST       #
# **************** #

require "test/unit"
class Test_find_peak_element < Test::Unit::TestCase
  def test_
    assert_equal 2, find_peak_element([1, 2, 3, 1])
    assert_equal 5, find_peak_element([1, 2, 1, 3, 5, 6, 4])
  end
end