Sort List
Sort a linked list in O(n log n) time using constant space complexity.
Analysis
- Function to merge two sorted lists
- Main function sortList:
- use slow/faster pointer to find mid node
- break lists into two lists
- sort 1st half and second half
- merge two lists
Code
public class Solution {
public ListNode sortList(ListNode head) {
if(head==null || head.next==null) {return head;}
ListNode mid = findMid(head);
ListNode rhead = mid.next;
ListNode lhead = head;
mid.next = null;
lhead = sortList(lhead);
rhead = sortList(rhead);
ListNode res = mergeLists(lhead, rhead);
return res;
}
private ListNode findMid(ListNode head){
if (head == null || head.next==null) {return head;}
ListNode slow = head, fast = head.next;
while (fast!=null && fast.next!=null){
slow = slow.next;
fast = fast.next.next;
}
return slow;
}
private ListNode mergeLists(ListNode l1, ListNode l2){
ListNode dummy = new ListNode(0);
ListNode curr = dummy;
while( (l1!=null) && (l2!=null)){
if(l1.val>l2.val){
curr.next = l2;
l2 = l2.next;
}
else {
curr.next = l1;
l1 = l1.next;
}
curr=curr.next;
}
curr.next = (l1!=null)?l1:l2;
return dummy.next;
}
}
Note
- Merge sort要掌握好
- Merge 2 lists要掌握好
- Find mid要掌握好
Reference