C++ Solution to Coding Challenge 19. Remove Nth Node From End of List
Using the two-pointer technique to remove a node in a linked list.
Problem statement
Given the head of a linked list, remove the n-th node from the end of the list and return its head.
Example 1
Input: head = [1,2,3,4,5], n = 2
Output: [1,2,3,5]
Example 2
Input: head = [1], n = 1
Output: []
Example 3
Input: head = [1,2], n = 1
Output: [1]
Constraints
The number of nodes in the list is
sz.1 <= sz <= 30.0 <= Node.val <= 100.1 <= n <= sz.
Follow up: Could you do this in one pass?
Solution 1: Store the nodes
Code
#include <iostream>
#include <vector>
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) {}
};
using namespace std;
ListNode* removeNthFromEnd(ListNode* head, int n) {
vector<ListNode*> nodes;
ListNode* node = head;
while (node)
{
nodes.push_back(node);
node = node->next;
}
node = nodes[nodes.size() - n];
if (node == head) {
head = node->next;
} else {
ListNode* pre = nodes[nodes.size() - n - 1];
pre->next = node->next;
}
return head;
}
void printList(ListNode *head) {
ListNode* node = head;
cout << "[";
while (node) {
cout << node->val << ",";
node = node->next;
}
cout << "]\n";
}
int main() {
ListNode five(5);
ListNode four(4, &five);
ListNode three(3, &four);
ListNode two(2, &three);
ListNode one(1, &two);
auto head = removeNthFromEnd(&one, 2);
printList(head);
head = removeNthFromEnd(&five, 1);
printList(head);
head = removeNthFromEnd(&four, 1);
printList(head);
}
Output:
[1,2,3,5,]
[]
[4,]
Complexity
Runtime:
O(sz), whereszis the number of nodes in the list.Extra space:
O(sz).
Solution 2: Two pointers
The distance between the removed node and the end (nullptr) of the list is always n.
You can apply the two-pointer technique as follows.
Let the slower runner start after the faster one n nodes. Then when the faster reaches the end of the list, the slower reaches the node to be removed.
Code
#include <iostream>
#include <vector>
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) {}
};
using namespace std;
ListNode* removeNthFromEnd(ListNode* head, int n) {
ListNode* fast = head;
for (int i = 0; i < n; i++) {
fast = fast->next;
}
if (fast == nullptr) {
return head->next;
}
ListNode* slow = head;
while (fast->next)
{
slow = slow->next;
fast = fast->next;
}
slow->next = slow->next->next;
return head;
}
void printList(ListNode *head) {
ListNode* node = head;
cout << "[";
while (node) {
cout << node->val << ",";
node = node->next;
}
cout << "]\n";
}
int main() {
ListNode five(5);
ListNode four(4, &five);
ListNode three(3, &four);
ListNode two(2, &three);
ListNode one(1, &two);
auto head = removeNthFromEnd(&one, 2);
printList(head);
head = removeNthFromEnd(&five, 1);
printList(head);
head = removeNthFromEnd(&four, 1);
printList(head);
}
Output:
[1,2,3,5,]
[]
[4,]
Complexity
Runtime:
O(sz), whereszis the number of nodes in the list.Extra space:
O(1).