147. Insertion Sort List

147. Insertion Sort List Medium

Sort a linked list using insertion sort.

A graphical example of insertion sort. The partial sorted list (black) initially contains only the first element in the list.
With each iteration one element (red) is removed from the input data and inserted in-place into the sorted list

Algorithm of Insertion Sort:

1. Insertion sort iterates, consuming one input element each repetition, and growing a sorted output list.
2. At each iteration, insertion sort removes one element from the input data, finds the location it belongs within the sorted list, and inserts it there.
3. It repeats until no input elements remain.

Example 1:

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Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

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Input: -1->5->3->4->0
Output: -1->0->3->4->5

思路：

思路一：插入排序

按照插入排序的思想，要注意的点：

• 不要断链
• 每一次都是从头开始进行比较

代码：

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public ListNode insertionSortList(ListNode head) {
    if (head == null || head.next == null) return head;

    ListNode res = new ListNode(0);
    ListNode pre = res;
    ListNode cur = head;
    ListNode next = null;
    while (cur != null) {
        next = cur.next;

        while (pre.next != null && pre.next.val < cur.val) {
            pre = pre.next;
        }
        
        cur.next = pre.next;
        pre.next = cur;
        // 从头开始
        pre = res;
        cur = next;
    }
    return res.next;
}

时间复杂度：O(n^2)

空间复杂度：O(1)