Leetcode Problem 162 Find Peak Element.txt

162. Find Peak Element


A peak element is an element that is greater than its neighbors.

Given an input array nums, where nums[i] ≠ nums[i+1], find a peak element and return its index.

The array may contain multiple peaks, in that case return the index to any one of the peaks is fine.

You may imagine that nums[-1] = nums[n] = -∞.

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: 1 or 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.
Note:

Your solution should be in logarithmic complexity.


Hint to solve: Use binary search.

public class Solution
{
    
    public int FindPeakElement(int[] nums) 
    {
        if(nums.Count() == 1)
            return 0;
        
        int start = 0;
        int end = nums.Count() - 1;
        
        while(start <= end)
        {
            int mid = (end+start)/2;
            
            if(mid-1 >= 0 && mid+1 < nums.Count() && nums[mid] > nums[mid-1] && nums[mid] > nums[mid+1])
            {
                return mid;
            }
            else if(mid+1 < nums.Count()-1 && nums[mid+1] > nums[mid])
            {
                start = mid+1;
                //Console.WriteLine("Start: "+ start);
            }
            else
            {
                end = mid-1;
                //Console.WriteLine("End: "+ end);
            }
        }
        
        //Compare the for the edge case. [2,1]. Because there is -infinity both sides.
        if(nums[0] > nums[1])
            return 0;
        else
            return nums.Count() - 1;
    }
}