C program to Find Turning Number in an Array

Write a C program to find turning number in an array. The problem is an interview questions. You have to find maximum number which increases at a certain index in an array and immediately decreases.

For example if the given array is {1, 2, 3, 4, 11, 8, 7, 6} , the turning number is 11. In Another example {1, 2, 3, 4, 11, 8, 7, 6,9,10,11,12,3,4} turning number is 12.

C program to Find Turning Number in an Array

#include <stdio.h>

int findTurningNumber(int *arr, int size)
{
   int i;
   int maximum = 0;
   for (i = 0; i < size; i++)
   {
       if (arr[i] > arr[i+1])
       {
          if( arr[i] > maximum)
               maximum = arr[i];
       }
   }
   return maximum;
}

int arr[] = {1, 2, 3, 4, 11, 8, 7, 6,9,10,11,12,3,4};

int main()
{
   int size = sizeof(arr)/sizeof(arr[0]);
   printf("The turning number is %d\n", findTurningNumber(arr, size));
   return 0;
}

Compile and run on Linux

bosch@bosch-Inspiron-N5050:~/myproject/sample$ gcc turning.c -o turning
bosch@bosch-Inspiron-N5050:~/myproject/sample$ ./turning 
The turning number is 12

The above implementation has time complexity : O(n)

More efficient way to write C program to Find Turning Number in an Array

The above solution can be modified to reduce time complexity of O(n) to time complexity of O(logn).

#include <stdio.h>
#include<limits.h>

int max = INT_MIN;

void findTurningNumber(int arr[], int start, int end) {

	// recursion end condition
	if (start == end) {
		return;
	}

	if ((start == end - 1) && arr[start] >= arr[end] && max < arr[start]) {
			max = arr[start];
	}

	if ((start == end -1) && arr[start] < arr[end] && max < arr[end]) {
			max = arr[end];
	}

	int mid = (start + end) / 2;
	if (arr[mid] > arr[mid + 1] && arr[mid] > arr[mid - 1] && max < arr[mid]) {
			max = arr[mid];
	}

	if (arr[mid] > arr[mid + 1] && arr[mid] < arr[mid - 1])
		findTurningNumber(arr, start, mid - 1);
	else
		findTurningNumber(arr, mid + 1, end);
}

int arr[] = { 1, 2, 3, 4, 9, 8, 7, 6, 9, 10, 11, 12, 3, 4 };

int main() {
	int size = sizeof(arr) / sizeof(arr[0]);
	findTurningNumber(arr, 0, size);
	printf("The turning number is %d\n", max);
	return 0;
}

Ref:

https://gist.github.com/luckypapa/2240d5839795d380b1df