Insertion Sort List | Leetcode

Sort a linked list using insertion sort.

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Solution: For each node in the given linked list, insert it in order into a new linked list. Not easy to get the logic right the first time one writes the code.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode insertionSortList(ListNode head) {
        if (head == null) return null;

        // Start off a new list reusing the same nodes.        
        ListNode newHead = head;
        head = head.next;
        newHead.next = null;
        
        // Insert each node in order.
        while (head != null) {
            // head.next will be overwritten, so save it first.
            ListNode next = head.next;
            newHead = insertInOrder(newHead, head);
            head = next;
        }
        
        return newHead;
    }
    
    private ListNode insertInOrder(ListNode head, ListNode curr) {
        // Find position at which we should insert curr.
        ListNode prev = null;
        ListNode node = head;
        while (node != null && curr.val > node.val) {
            prev = node;
            node = node.next;
        }
        
        // We might need to insert as the head of the list.
        if (prev == null) {
            curr.next = head;
            return curr;
        }
        
        // Insert as per normal inside the list.
        curr.next = prev.next;
        prev.next = curr;
        return head;
    }
}