148. Sort List

README.md

@@ -76,6 +76,7 @@ continually updating 😃.
 * [83. Remove Duplicates from Sorted List](src/0083_remove_duplicates_from_sorted_list/rdfsl.go)
 * [86. Partition List](src/0086_partition_list/partition_list.go)   *`two pointers`*
 * [92. Reverse Linked List II](src/0092_reverse_linked_list_2/reverse_linked_list2.go)
+* [148. Sort List](src/148_Sort_List/sortlist.go)   *`sort`*
 * [203. Remove Linked List Elements](src/0203_remove_linked_list_elements/remove_linked_list_elements.go)
 * [206. Reverse Linked List](src/0206_reverse_linked_list/reverse_linked_list.go)
 * [237. Delete Node in a Linked List](src/0237_delete_node_in_a_linked_list/dniall.go)

src/148_Sort_List/sortlist.go

@@ -0,0 +1,53 @@
+/*
+148. Sort List
+https://leetcode.com/problems/sort-list/
+*/
+// time: 2019-03-04
+
+package sortlist
+
+// ListNode Definition for singly-linked list.
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+// merge sort
+// time complexity: O(n * log(n))
+// using recursion, the system stack is used.
+func sortList(head *ListNode) *ListNode {
+	if head == nil || head.Next == nil {
+		return head
+	}
+
+	prev, slow, fast := head, head, head
+	for fast != nil && fast.Next != nil {
+		prev = slow
+		slow = slow.Next
+		fast = fast.Next.Next
+	}
+	prev.Next = nil
+	return merge(sortList(head), sortList(slow))
+}
+
+func merge(headA, headB *ListNode) *ListNode {
+	dummy := &ListNode{}
+	tail := dummy
+
+	for headA != nil && headB != nil {
+		if headA.Val > headB.Val {
+			tail.Next = headB
+			headB = headB.Next
+		} else {
+			tail.Next = headA
+			headA = headA.Next
+		}
+		tail = tail.Next
+	}
+	if headA != nil {
+		tail.Next = headA
+	} else {
+		tail.Next = headB
+	}
+	return dummy.Next
+}

src/148_Sort_List/sortlist_test.go

@@ -0,0 +1,24 @@
+package sortlist
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestSortList(t *testing.T) {
+	head := createSingleLinkedList([]int{-1, 5, 3, 4, 0})
+	expected := createSingleLinkedList([]int{-1, 0, 3, 4, 5})
+	if res := sortList(head); !reflect.DeepEqual(res, expected) {
+		t.Errorf("expected %v, got %v", expected, res)
+	}
+}
+
+func createSingleLinkedList(nums []int) *ListNode {
+	dummy := ListNode{}
+	cur := &dummy
+	for _, val := range nums {
+		cur.Next = &ListNode{Val: val}
+		cur = cur.Next
+	}
+	return dummy.Next
+}

src/README.md

@@ -60,6 +60,7 @@
 |0125|[Valid Palindrome](0125_valid_palindrome/valid_palindrome.go)|Easy||
 |0136|[136. Single Number](0136_single_number/single_number.go)|Easy|*`hash table;`* *`bit manipulation`*|
 |0144|[144. Binary Tree Preorder Traversal](0144_binary_tree_preorder_traversal/binary_tree_preorder_traversal.go)|Medium|*`binary tree`*|
+|0148|[148. Sort List](148_Sort_List/sortlist.go)|Medium|*`sort;`* *`linked list`*|
 |0150|[150. Evaluate Reverse Polish Notation](0150_evaluate_reverse_polish_notation/evaluate_reverse_polish_notation.go)|Medium|*`stack`*|
 |0153|[153. Find Minimum in Rotated Sorted Array](0153_find_minimum_in_rotated_sorted_array/fmirsa.go)|Medium|*`binary search`*|
 |0155|[155. Min Stack](0155_min_stack/min_stack.go)|Easy|*`stack`*|