Adds linked list insert solution

Author: omonimus1

c++/left_index.cpp

@@ -0,0 +1,12 @@
+// https://practice.geeksforgeeks.org/problems/left-index-1587115620/1/?track=SPCF-Searching&batchId=154
+int leftIndex(int N, int arr[], int X){
+    if(arr[N-1] < X)
+        return -1;
+        
+    for(int i =0; i < N; i++)
+    {
+        if(arr[i] == X)
+            return i;
+    }
+    return -1; 
+}

c++/linked-list-insert.cpp

@@ -0,0 +1,36 @@
+// https://practice.geeksforgeeks.org/problems/linked-list-insertion-1587115620/1/?track=PC-W5-LL&batchId=154/*Structure of the linked list node is as
+struct Node {
+  int data;
+  struct Node * next;
+  Node(int x) {
+    data = x;
+    next = NULL;
+  }
+}; */
+
+// function inserts the data in front of the list
+Node *insertAtBegining(Node *head, int newData) {
+   Node *new_node = new Node(newData);
+   if(head == NULL)
+        return new_node;
+    else
+    {
+        new_node ->next = head;
+        return new_node;
+    }
+}
+
+
+// function appends the data at the end of the list
+Node *insertAtEnd(Node *head, int newData)  {
+    Node *new_node = new Node(newData);
+    if(head == NULL)
+      return new_node;
+      
+    Node *current = head;
+    while(current->next != NULL)
+        current = current->next;
+        
+    current->next = new_node;
+    return head;
+}