0019-remove-nth-node-from-end-of-list.cpp

/*
 Given head of a linked list, remove nth node from end of list
 Ex. head = [1,2,3,4,5], n = 2 -> [1,2,3,5]

 Create 2 pointers "n" apart, iterate until end, will be at nth

 Time: O(n)
 Space: O(1)
*/

/**
 * 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) {}
 * };
*/
classSolution {
public:
 ListNode* removeNthFromEnd(ListNode* head, int n) {
if (head->next == NULL) {
returnNULL;
 }

 ListNode* slow = head;
 ListNode* fast = head;

while (n > 0) {
 fast = fast->next;
 n--;
 }

if (fast == NULL) {
return head->next;
 }

while (fast->next != NULL) {
 slow = slow->next;
 fast = fast->next;
 }

 slow->next = slow->next->next;
return head;
 }
};