Find Element in Array Forming Strictly Decreasing and Increasing Sequence in Python

Suppose we have an array of positive numbers; we have to check a point/item up to which items create a strictly decreasing sequence first followed by a sequence of strictly increasing integers. These are the following properties: We have to keep in mind that the sequences must be of minimum length 2

Also, we have taken care that the last value of the decreasing sequence is the first value of the increasing sequence.

So, if the input is like {5, 4, 3, 4}, then the output will be 3, as {5, 4, 3} is strictly decreasing then {3, 4} is strictly increasing.

To solve this, we will follow these steps −

• increase := 1, decrease := 1
• n := size of array
• for i in range 1 to n, do
  • if array[i] < array[i-1], then
    • if increase is same as 1, then
      • decrease := decrease + 1
    • otherwise,
      • return -1
  • otherwise when array[i] > array[i-1] is non-zero, then
    • if increase is same as 1, then
      • pt := array[i-1]
    • if decrease >= 2, then
      • increase := increase + 1
    • otherwise,
      • return -1
  • otherwise when array[i] is same as array[i-1], then
    • return -1
• if increase >= 2 and decrease >= 2, then
  • return pt
• otherwise,
  • return -1

Example

Let us see the following implementation to get better understanding −

 Live Demo

def search_element(array):
   increase = 1
   decrease = 1
   n = len(array)
   for i in range(1, n):
      if(array[i] < array[i-1]):
         if increase == 1:
            decrease = decrease + 1
         else:
            return -1
      elif(array[i] > array[i-1]):
         if increase == 1:
            pt = array[i-1]
         if decrease >= 2:
            increase = increase + 1
         else:
            return -1
      elif(array[i] == array[i-1]):
         return -1
   if(increase >= 2 and decrease >= 2):
      return pt
   else:
      return -1
array = [5,4,3,4]
element = search_element(array)
print(element)

Input

[5,4,3,4]

Output

3