From dd036c8129334788fe8eabfeb2602c5360654b8e Mon Sep 17 00:00:00 2001
From: kunal jain <kkunal95jjain@gmail.com>
Date: Thu, 21 Apr 2022 16:05:49 -0400
Subject: [PATCH] insert and remove nodes in a linked list at a given position

---
 linked_list_more.c | 117 +++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 112 insertions(+), 5 deletions(-)

diff --git a/linked_list_more.c b/linked_list_more.c
index 9e01d5d..356da07 100644
--- a/linked_list_more.c
+++ b/linked_list_more.c
@@ -57,24 +57,126 @@ void printLinkedListReverse (struct NODE* last) {
     }
 }
 
-void removeNode(struct NODE ** addr_of_first, int position) {
+// Insert node at a given position in linked list
+void insertNode(struct NODE ** addr_of_first, struct NODE ** addr_of_last, int position, char value) {
+    // addr_of_first: address of the pointer to 1st node in the list
+    // position: position to insert the node
+    // value: value to insert
+
+    // Create node to insert using malloc
+    struct NODE * node_to_insert = malloc(sizeof(struct NODE));
+    (*node_to_insert).value = value;
+    (*node_to_insert).next = NULL;
+    (*node_to_insert).prev = NULL;
+
+    // Create node first, to hold value at address of first node
+    struct NODE * first = NULL;
+    first = *addr_of_first;
+    
+    // Create node current to keep track of node at current position
+    // Initialize current node with first node.
+    struct NODE * current = NULL;
+    current = first;
+
+    // Create node target to keep track of node at position - 1.
+    // Insert node next to the target node
+    // Initialize target with current.prev 
+    struct NODE * target = NULL;
+    target = (*current).prev;
+
+    int i;
+    // Run a loop to reach the position where we want to insert
+    for (i = 1; i < position; i++) {
+        // Set target = current and
+        // current = current.next, until we reach insertion position
+        target = current;
+        current = (*current).next;
+    }
+
+    // If target == NULL 
+    if (target == NULL) {
+        // insert node at the first position
+        // set node_to_insert.next = current
+        (*node_to_insert).next = current;
+        (*current).prev = node_to_insert;
+
+        // update value at address of first node
+        *addr_of_first = node_to_insert;
+    }
+    
+    // If current == NULL
+    else if (current == NULL) {
+        // insert node at last position
+        // set target.next = node_to_insert
+        (*target).next = node_to_insert;
+        (*node_to_insert).prev = target;
+
+        // update value at address of last node
+        *addr_of_last = node_to_insert;
+    }
+    
+    // else
+    else {
+        // insert node between target and current node.
+        // set forward and backward link between 
+        // target and node_to_insert
+        (*target).next = node_to_insert;
+        (*node_to_insert).prev = target;
+        // similarly set forward and backward link between 
+        // node_to_insert and current
+        (*node_to_insert).next = current;
+        (*current).prev = node_to_insert;
+    }
+    
+}
+
+void removeNode(struct NODE ** addr_of_first, struct NODE ** addr_of_last, int position) {
+    // Create a struct node. We will delete this node.
     struct NODE * node_to_remove = NULL;
 
+    // Initialize node_to_remove with value at address of first node
     node_to_remove = *addr_of_first;
+
+    // Run a loop to reach to the position that we need to remove
     int i;
     for (i  = 1; i < position; i++) {
+        // Reach node_to_remove position and
+        // set node_to_remove to node_to_remove.next
         node_to_remove = (*node_to_remove).next;
     }
+
+    // get previous and next node of node_to_remove
     struct NODE * prev_node = (*node_to_remove).prev;
     struct NODE * next_node = (*node_to_remove).next;
 
-    (*prev_node).next = next_node;
-    (*next_node).prev = prev_node;
+    // Unlink node_to_remove.
+    // Reset next value of prev_node and 
+    // prev value of next_node.
+    if (prev_node == NULL) {
+        // Reset prev value of next_node
+        (*next_node).prev = NULL;
+
+        // Update value at address of first node
+        *addr_of_first = next_node;
+    }
+    else if (next_node == NULL) {
+        // Reset next value of prev_node
+        (*prev_node).next = next_node;
+
+        // Update value at address of last node
+        *addr_of_last = prev_node;
+    }
+    else {
+        // Reset next value of prev_node
+        (*prev_node).next = next_node;
+        // Reset prev value of next_node
+        (*next_node).prev = prev_node;
+    }
 
+    // Release memory using free function
     free(node_to_remove);
 }
 
-
 // Append new node in a linked list in much less time and computation.
 // Instead of going to the first element and then going through
 // all the elements using node.next to reach last element, 
@@ -148,7 +250,12 @@ int main(void) {
 
     printf("\n");
 
-    removeNode(&first, 2);
+    removeNode(&first, &last, 5);
+
+    printLinkedList(first);
+    printf("\n");
+
+    insertNode(&first, &last, 5, '7');
 
     printLinkedList(first);
     printf("\n");