Delete all the nodes from the list that are greater than x

Given a linked list, the problem is to delete all the nodes from the list that are greater than the specified value x.

Examples:

Input : list: 7->3->4->8->5->1
        x = 6
Output : 3->4->5->1

Input : list: 1->8->7->3->7->10
        x = 7
Output : 1->7->3->7

Source: Microsoft Interview Experience | Set 169.



Approach: This is mainly a variation of the post which deletes first occurrence of a given key.

We need to first check for all occurrences at head node which are greater than ‘x’, delete them and change the head node appropriately. Then we need to check for all occurrences inside a loop and delete them one by one.

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// C++ implementation to delete all the nodes from the list
// that are greater than the specified value x
#include <bits/stdc++.h>
  
using namespace std;
  
// structure of a node
struct Node {
    int data;
    Node* next;
};
  
// function to get a new node
Node* getNode(int data)
{
    Node* newNode = new Node;
    newNode->data = data;
    newNode->next = NULL;
    return newNode;
}
  
// function to delete all the nodes from the list
// that are greater than the specified value x
void deleteGreaterNodes(Node** head_ref, int x)
{
    Node *temp = *head_ref, *prev;
  
    // If head node itself holds the value greater than 'x'
    while (temp != NULL && temp->data > x) {
        *head_ref = temp->next; // Changed head
        free(temp); // free old head
        temp = *head_ref; // Change temp
    }
  
    // Delete occurrences other than head
    while (temp != NULL) {
  
        // Search for the node to be deleted, 
        // keep track of the previous node as we 
        // need to change 'prev->next'
        while (temp != NULL && temp->data <= x) {
            prev = temp;
            temp = temp->next;
        }
  
        // If required value node was not present
        // in linked list
        if (temp == NULL)
            return;
  
        // Unlink the node from linked list
        prev->next = temp->next;
  
        delete temp; // Free memory
  
        // Update Temp for next iteration of 
        // outer loop
        temp = prev->next;
    }
}
  
// function to a print a linked list
void printList(Node* head)
{
    while (head) {
        cout << head->data << " ";
        head = head->next;
    }
}
  
// Driver program to test above
int main()
{
    // Create list: 7->3->4->8->5->1
    Node* head = getNode(7);
    head->next = getNode(3);
    head->next->next = getNode(4);
    head->next->next->next = getNode(8);
    head->next->next->next->next = getNode(5);
    head->next->next->next->next->next = getNode(1);
  
    int x = 6;
  
    cout << "Original List: ";
    printList(head);
  
    deleteGreaterNodes(&head, x);
  
    cout << "\nModified List: ";
    printList(head);
  
    return 0;
}

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Output:

Original List: 7 3 4 8 5 1 
Modified List: 3 4 5 1

Time Complexity: O(n).



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