2487_Remove_Nodes_From_Linked_List.cpp

/*
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.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* removeNodes(ListNode* head) {
        ListNode* cur = head;
        stack<ListNode*> stack;
        
        while (cur != nullptr) {
            while (!stack.empty() && stack.top()->val < cur->val) {
                stack.pop();
            }
            stack.push(cur);
            cur = cur->next;
        }
        
        ListNode* nxt = nullptr;
        while (!stack.empty()) {
            cur = stack.top();
            stack.pop();
            cur->next = nxt;
            nxt = cur;
        }
        
        return cur;
    }
};