Peak Index in a Mountain Array

An array arr a mountain if the following properties hold:

  • arr.length >= 3
  • There exists some i with 0 < i < arr.length - 1 such that:
    • arr[0] < arr[1] < ... < arr[i - 1] < arr[i]
    • arr[i] > arr[i + 1] > ... > arr[arr.length - 1]

Given a mountain array arr, return the index i such that arr[0] < arr[1] < ... < arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].

You must solve it in O(log(arr.length)) time complexity.

 

Example 1:

Input: arr = [0,1,0]
Output: 1

Example 2:

Input: arr = [0,2,1,0]
Output: 1

Example 3:

Input: arr = [0,10,5,2]
Output: 1

 

Constraints:

  • 3 <= arr.length <= 105
  • 0 <= arr[i] <= 106
  • arr is guaranteed to be a mountain array.

C++ Program Solution for Peak Index in a Mountain Array

class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) {
       
        int s=0;
        int n=arr.size();
        int e=n-1;
        int mid= s+((e-s)/2);
        while(s<e)
        {
           if(arr[mid]<arr[mid+1])
           {
               s=mid+1;
               
           }
            else
            {
                e=mid;
               
            }
            mid=s+((e-s)/2);
        }
       
        return s;
    }
   
};

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