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| 1 | +/* |
| 2 | + * @lc app=leetcode.cn id=148 lang=java |
| 3 | + * |
| 4 | + * [148] 排序链表 |
| 5 | + */ |
| 6 | + |
| 7 | +// @lc code=start |
| 8 | +/** |
| 9 | + * Definition for singly-linked list. |
| 10 | + * public class ListNode { |
| 11 | + * int val; |
| 12 | + * ListNode next; |
| 13 | + * ListNode() {} |
| 14 | + * ListNode(int val) { this.val = val; } |
| 15 | + * ListNode(int val, ListNode next) { this.val = val; this.next = next; } |
| 16 | + * } |
| 17 | + */ |
| 18 | +class Solution { |
| 19 | + public ListNode sortList(ListNode head) { |
| 20 | + return mergeSort(head); |
| 21 | + } |
| 22 | + |
| 23 | + public ListNode findMiddle(ListNode node) { |
| 24 | + ListNode slow = node; |
| 25 | + ListNode fast = node.next; |
| 26 | + |
| 27 | + while(fast != null && fast.next != null) { |
| 28 | + fast = fast.next.next; |
| 29 | + slow = slow.next; |
| 30 | + } |
| 31 | + |
| 32 | + return slow; |
| 33 | + } |
| 34 | + |
| 35 | + public ListNode mergeTwoLists(ListNode node1, ListNode node2) { |
| 36 | + ListNode dummy = new ListNode(); |
| 37 | + ListNode head = dummy; |
| 38 | + |
| 39 | + while(node1 != null && node2 != null) { |
| 40 | + if(node1.val < node2.val) { |
| 41 | + head.next = node1; |
| 42 | + node1 = node1.next; |
| 43 | + } else { |
| 44 | + head.next = node2; |
| 45 | + node2 = node2.next; |
| 46 | + } |
| 47 | + |
| 48 | + head = head.next; |
| 49 | + } |
| 50 | + |
| 51 | + if(node1 != null) { |
| 52 | + head.next = node1; |
| 53 | + } |
| 54 | + |
| 55 | + if(node2 != null) { |
| 56 | + head.next = node2; |
| 57 | + } |
| 58 | + |
| 59 | + return dummy.next; |
| 60 | + } |
| 61 | + |
| 62 | + public ListNode mergeSort(ListNode node) { |
| 63 | + if(node == null || node.next == null) { |
| 64 | + return node; |
| 65 | + } |
| 66 | + |
| 67 | + ListNode middleNode = findMiddle(node); |
| 68 | + ListNode middleNodeNext = middleNode.next; |
| 69 | + middleNode.next = null; |
| 70 | + |
| 71 | + ListNode node1 = mergeSort(node); |
| 72 | + ListNode node2 = mergeSort(middleNodeNext); |
| 73 | + |
| 74 | + return mergeTwoLists(node1, node2); |
| 75 | + } |
| 76 | +} |
| 77 | +// @lc code=end |
| 78 | + |
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