Program to insert new element into a linked list before the given position in Python

Inserting a new element into a linked list at a specific position is a common operation in data structures. This involves creating a new node and adjusting the links between existing nodes to maintain the list structure.

So, if the input is like nums = [1,5,3,6,8] pos = 3 val = 7, then the output will be [1,5,3,7,6,8].

Algorithm

To solve this, we will follow these steps −

• new := create a linked list node with value same as val

• if pos is same as 0, then

  • next of new := list_head

  • return new

• temp := list_head

• while temp is not null and pos is not same as 1, do

  • temp := next of temp

  • pos := pos - 1

• next of new := next of temp

• next of temp := new

• return list_head

Implementation

Let us see the following implementation to get better understanding −

class ListNode:
    def __init__(self, data, next=None):
        self.val = data
        self.next = next

def make_list(elements):
    head = ListNode(elements[0])
    for element in elements[1:]:
        ptr = head
        while ptr.next:
            ptr = ptr.next
        ptr.next = ListNode(element)
    return head

def print_list(head):
    ptr = head
    result = []
    while ptr:
        result.append(str(ptr.val))
        ptr = ptr.next
    return '[' + ', '.join(result) + ']'

def solve(list_head, pos, val):
    new = ListNode(val)
    if pos == 0:
        new.next = list_head
        return new
    
    temp = list_head
    while temp and pos != 1:
        temp = temp.next
        pos -= 1
    
    new.next = temp.next
    temp.next = new
    return list_head

# Example usage
nums = [1, 5, 3, 6, 8]
pos = 3
val = 7

list_head = make_list(nums)
list_head = solve(list_head, pos, val)
print(print_list(list_head))

[1, 5, 3, 7, 6, 8]

How It Works

The algorithm works by traversing the linked list until we reach the position where we need to insert the new element. We maintain a pointer that stops at the node just before the insertion point, then adjust the next pointers to include the new node in the chain.

Key Points

• For position 0, we insert at the beginning and update the head

• For other positions, we traverse to position-1 and insert the new node

• The new node's next pointer points to the current node at that position

• The previous node's next pointer points to the new node

Conclusion

Inserting elements into a linked list requires careful manipulation of next pointers. The key is to find the correct position and update the links to maintain the list structure while preserving all existing elements.