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

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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)

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