Find Maximum Element in an Array First Increasing Then Decreasing in C++

Suppose we have one array, which is initially increasing then decreasing. We have to find the max value in the array. So if the array elements are like A = [8, 10, 20, 80, 100, 250, 450, 100, 3, 2, 1], then output will be 500.

We can use the binary search to solve this. There are three conditions −

• When mid is greater than both of its adjacent elements, then mid is maximum
• If mid is greater than the next element, but smaller than previous element, then max lies on the left side of mid.
• If mid element is smaller than the next element, but greater than previous element, then max lies on the right side of mid.

Example

 Live Demo

#include<iostream>
using namespace std;
int getMaxElement(int array[], int left, int right) {
   if (left == right)
      return array[left];
   if ((right == left + 1) && array[left] >= array[right])
      return array[left];
   if ((right == left + 1) && array[left] < array[right])
      return array[right];
   int mid = (left + right)/2;
   if ( array[mid] > array[mid + 1] && array[mid] > array[mid - 1])
      return array[mid];
   if (array[mid] > array[mid + 1] && array[mid] < array[mid - 1])
      return getMaxElement(array, left, mid-1);
   else
      return getMaxElement(array, mid + 1, right);
}
int main() {
   int array[] = {8, 10, 20, 80, 100, 250, 450, 100, 3, 2, 1};
   int n = sizeof(array)/sizeof(array[0]);
   cout << "The maximum element is: " << getMaxElement(array, 0, n-1);
}

Output

The maximum element is: 450