Find whether given Array is in form of a mountain or not

Given an array arr[]. The task is to check whether it is a mountain array or not. A mountain array is an array of length at least 3 with elements strictly increasing from starting till an index i, and then strictly decreasing from index i to last index. More formally arr[0] < arr[1] < arr[i] >arr[i+1] > arr[i+2] > arr[N-1]. 

Examples

Input: arr[] = {4, 4, 3, 2, 1}
Output: false

Input: arr = {1, 2, 3, 4, 9, 8, 7, 6, 5}
Output: true

Approach: This problem can be solved by traversing the array in two parts. Follow the steps below to solve the given problem.

• First, traverse from start till the index where the current element becomes smaller than its immediate previous one.
• Then from this index, traverse till the last index and check if the current element is strictly smaller than the previous one.
• If both the conditions are true, return true.
• Else at any point, the condition fails, return false.

Below is the implementation of the above approach.

C++

// C++ program for above approach
#include <bits/stdc++.h>
using namespace std;

// Function to check if the given array
// is mountain array or not
bool isMountainArray(vector<int>& arr)
{
    if (arr.size() < 3)
        return false;
    int flag = 0, i = 0;
    for (i = 1; i < arr.size(); i++)
        if (arr[i] <= arr[i - 1])
            break;

    if (i == arr.size() || i == 1)
        return false;

    for (; i < arr.size(); i++)
        if (arr[i] >= arr[i - 1])
            break;
    return i == arr.size();
}

// Driver Code
int main()
{
    vector<int> arr
        = { 1, 2, 3, 4, 9,
            8, 7, 6, 5 };
    cout << (isMountainArray(arr)
                 ? "true"
                 : "false");
    return 0;
}

Java

// Java program for the above approach
import java.io.*;
import java.lang.*;
import java.util.*;

class GFG {

// Function to check if the given array
// is mountain array or not
static Boolean isMountainArray(int arr[])
{
    if (arr.length < 3)
        return false;
    int flag = 0, i = 0;
    for (i = 1; i < arr.length; i++)
        if (arr[i] <= arr[i - 1])
            break;

    if (i == arr.length || i == 1)
        return false;

    for (; i < arr.length; i++)
        if (arr[i] >= arr[i - 1])
            break;
    return i == arr.length;
}

// Driver Code
    public static void main (String[] args) {
           int arr[] = { 1, 2, 3, 4, 9,
            8, 7, 6, 5 };
       System.out.println(isMountainArray(arr)
                 ? "true"
                 : "false");

    }
}

// This code is contributed by hrithikgarg03188.

Python3

# Python 3 program for above approach

# Function to check if the given array
# is mountain array or not
def isMountainArray(arr):

    if (len(arr) < 3):
        return False
    flag = 0
    i = 0
    for i in range(1, len(arr)):
        if (arr[i] <= arr[i - 1]):
            break

    if (i == len(arr) or i == 1):
        return False

    while i < len(arr):
        if (arr[i] >= arr[i - 1]):
            break
        i += 1
    return i == len(arr)

# Driver Code
if __name__ == "__main__":

    arr = [1, 2, 3, 4, 9,
           8, 7, 6, 5]
    if (isMountainArray(arr)):
        print("true")
    else:
        print("false")

        # This code is contributed by ukasp.

C#

// C# program for above approach
using System;
class GFG
{

  // Function to check if the given array
  // is mountain array or not
  static bool isMountainArray(int[] arr)
  {
    if (arr.Length < 3)
      return false;
    int i = 0;
    for (i = 1; i < arr.Length; i++)
      if (arr[i] <= arr[i - 1])
        break;

    if (i == arr.Length || i == 1)
      return false;

    for (; i < arr.Length; i++)
      if (arr[i] >= arr[i - 1])
        break;
    return i == arr.Length;
  }

  // Driver Code
  public static int Main()
  {
    int[] arr = { 1, 2, 3, 4, 9, 8, 7, 6, 5 };
    Console.Write(isMountainArray(arr) ? "true"
                  : "false");
    return 0;
  }
}

// This code is contributed by Taranpreet

Javascript

<script>
    // JavaScript program for above approach

    // Function to check if the given array
    // is mountain array or not
    const isMountainArray = (arr) => {
        if (arr.length < 3)
            return false;
        let flag = 0, i = 0;
        for (i = 1; i < arr.length; i++)
            if (arr[i] <= arr[i - 1])
                break;

        if (i == arr.length || i == 1)
            return false;

        for (; i < arr.length; i++)
            if (arr[i] >= arr[i - 1])
                break;
        return i == arr.length;
    }

    // Driver Code

    let arr = [1, 2, 3, 4, 9,
        8, 7, 6, 5];
    (isMountainArray(arr)
        ? document.write("true")
        : document.write("false"));

// This code is contributed by rakeshsahni

</script>

Output

true

Time Complexity: O(N) 
Auxiliary Space: O(1)