Difficulty: Medium
Sort a linked list in O(n log n) time using constant space complexity.
Example 1:
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| Input: 4->2->1->3
Output: 1->2->3->4
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Example 2:
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| Input: -1->5->3->4->0
Output: -1->0->3->4->5
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Solution
Language: Java
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class Solution {
public ListNode sortList(ListNode head) {
if (head == null) {
return head;
}
return sort(head);
}
private ListNode sort(ListNode head) {
if (head == null || head.next == null) {
return head;
}
ListNode slow = head;
ListNode fast = head;
while (fast.next != null && fast.next.next != null) {
fast = fast.next.next;
slow = slow.next;
}
fast = slow.next;
slow.next = null;
head = sort(head);
fast = sort(fast);
return merge(head, fast);
}
private ListNode merge(ListNode head1, ListNode head2) {
ListNode head;
if (head1.val < head2.val) {
head = head1;
head1 = head1.next;
} else {
head = head2;
head2 = head2.next;
}
ListNode cur = head;
while (head1 != null || head2 != null) {
if (head1 == null) {
cur.next = head2;
head2 = head2.next;
} else if (head2 == null) {
cur.next = head1;
head1 = head1.next;
} else if (head1.val < head2.val) {
cur.next = head1;
head1 = head1.next;
} else {
cur.next = head2;
head2 = head2.next;
}
cur = cur.next;
cur.next = null;
}
return head;
}
}
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