2487.RemoveNodes.cs

// 2487. Remove Nodes From Linked List

// You are given the head of a linked list.
// Remove every node which has a node with a greater value anywhere to the right side of it.
// Return the head of the modified linked list.

// Example 1:
// Input: head = [5,2,13,3,8]
// Output: [13,8]
// Explanation: The nodes that should be removed are 5, 2 and 3.
// - Node 13 is to the right of node 5.
// - Node 13 is to the right of node 2.
// - Node 8 is to the right of node 3.
// Example 2:
// Input: head = [1,1,1,1]
// Output: [1,1,1,1]
// Explanation: Every node has value 1, so no nodes are removed.

// Constraints:
// The number of the nodes in the given list is in the range [1, 105].
// 1 <= Node.val <= 105

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode RemoveNodes(ListNode head) {
        Stack<ListNode> st = new Stack<ListNode>();
        ListNode itr = head;
        while(itr != null){
            while(st.Any() && itr.val > st.Peek().val){
                st.Pop();
            }
            st.Push(itr);
            itr = itr.next;
        }
        itr = null;
        while(st.Any()){
            head = st.Pop();
            head.next = itr;
            itr = head;
        }
        return head;
    }
}