Remove Nth Node From End of List

Description

Given the head of a linked list, remove the nth 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(javascript)

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} n
 * @return {ListNode}
 */
var removeNthFromEnd = function(head, n) {
    // move currentNode n steps into list
    let currentNode = head;
    
    for ( let i = 0; i < n; i++) {
        currentNode = currentNode.next;
    }
    
    if (currentNode == null) {
        return head.next;
    }
    
    // move both pointers until currentNode reaches the end of list
    let nodeBeforeRemoved = head;
    
    while(currentNode.next !== null) {
        currentNode = currentNode.next;
        nodeBeforeRemoved = nodeBeforeRemoved.next;
    }
    
    nodeBeforeRemoved.next = nodeBeforeRemoved.next.next;
    
    return head;
    // Time = O(n)
    // Space = O(n)
    
//     let length = 0;
//     let currentNode = head;
    
//     // find length of list
//     while(currentNode !== null) {
//         currentNode = currentNode.next;
//         length++;
//     }
    
//     if (length == n) {
//         return head.next;
//     }
    
//     // find the node to remove
//     let nodeIndex = length - n 
//     let nodeBeforeRemovedIndex = nodeIndex - 1;
//     currentNode = head;
//     for(let i = 0; i < nodeBeforeRemovedIndex; i++) {
//         currentNode = currentNode.next;
//     }
//     currentNode.next = currentNode.next.next;
    
//     return head;
    
//     // Time = O(n)
//     // Space = O(1)
    
    
};