Python Tutorial

In this tutorial, we will write the Python program to find the greater element in the given list. A greater element means an element in the array is the nearest element on the right, greater than the current element. Let's see the problem statement.

Problem Statement

Given an array arr[] of size N, the task is to find the next greater element for each element of the array in the order of their appearance in the array. If there is no greater element of the current element, then next greater element for current element is -1.

Example -

Input: 
N = 4, arr[] = [1 3 2 4]
Output:
3 4 4 -1
Explanation:
In the array, the next larger element 
to 1 is 3 , 3 is 4 , 2 is 4 and for 4 ? 
since it doesn't exist, it is -1.

Solution -

We can solve it using the stack data- structure. Let's understand the following example.

Example -

def next_greater_element(arr):
    stack = []
    result = [-1] * len(arr)
    for i in range(len(arr)):
        while stack and arr[i] > arr[stack[-1]]:
            result[stack.pop()] = arr[i]
        stack.append(i)
    return result
print(next_greater_element([1, 3, 2, 4]))

Output:

[3, 4, 4, -1]

Explanation -

In the above code, we initialize an empty stack and a result list of size N, where each element is initially set to -1.

Next, we loop through the array arr[] from left to right. For each element arr[i], we compare it with the top element of the stack. If arr[i] is greater than the top element, we pop the index from the stack and set the corresponding element in the result list to arr[i]. We continue this process until either the stack is empty or the top element is greater than arr[i].

After processing each element in arr[], the result list will contain the next greater element for each element of the array in the order of their appearance in the array. If an element does not have a next greater element, its corresponding value in the result list will remain -1.

Method - 2: Brute-Force Approach

In this approach, we use the nested loop and compare each element with the elements to its right until a greater element is found. The algorithm is given below.

Algorithm -

Traverse the given array from left to right.
For each element arr[i], traverse the subarray arr[i+1] to arr[N-1] to find the next greater element.
If a greater element is found, assign it to the variable next greater and break the inner loop.
If no greater element is found, assign -1 to the variable next greater.
Print the value of next greater for each element.

Let's understand the following code.

Example -

# Function to find the next greater element for each element in the array
def next_greater_element(arr):
    # Traverse the array from left to right
    for i in range(len(arr)):
        # Initialize next greater to -1
        next_greater = -1
        # Traverse the subarray to the right of arr[i]
        for j in range(i+1, len(arr)):
            # If a greater element is found, assign it to next greater
            if arr[j] > arr[i]:
                next_greater = arr[j]
                break
        # Print the value of next greater
        print(next_greater, end=" ")
    print()
arr = [4, 5, 2, 25, 13, 8]
print(next_greater_element(arr))

Output: