I like to read this solution as a small machine: keep the useful information, throw away the noise. For 148. Sort List, the solution in this repository is mainly a greedy solution.
Guide
What?
The first job is to translate the English prompt into state, transition, and stopping conditions. Instead of trying to be clever immediately, read the code as a sequence of questions:
- What state are we keeping?
- How do we move from one state to the next?
- When do we know the answer is already determined?
For this file, the main tools are: greedy.
Guide
When?
This is the kind of solution you want when the problem has structure hiding inside a messy-looking input. The accepted code reduces that pressure by storing exactly the information that remains useful later.
The important function names to track are merge, sortList.
Guide
Why?
The code is doing bookkeeping so your brain does not have to keep the entire search space open at once.
- Sorting is used to make local choices comparable, so the later scan does not have to rediscover order.
- The final return is not magic; it is the invariant after the loops or recursion have finished doing their accounting.
Guide
How?
Walk through the solution in this order:
- Initialize the memory or helper structure.
- Process candidates in the order the invariant expects.
- Update the answer only when the current state is valid.
- Return the value that represents the fully processed input.
The most important competitive-programming habit here is to trust the invariant. Once the invariant is right, the loops become much less scary.
Guide
Complexity
- Time: O(n) to O(n log n), depending on the dominant loop or data structure operation
- Space: O(n) in the usual case for auxiliary containers or recursion
Guide
C++ Solution
Your submission
The accepted solution
solution.cpp
01//Runtime: 68 ms, faster than 32.71% of C++ online submissions for Sort List.
02//Memory Usage: 12.7 MB, less than 27.50% of C++ online submissions for Sort List.
03/**
04 * Definition for singly-linked list.
05 * struct ListNode {
06 * int val;
07 * ListNode *next;
08 * ListNode(int x) : val(x), next(NULL) {}
09 * };
10 */
11class Solution {
12public:
13// void merge(ListNode* a, ListNode* b){
14// ListNode *newHead = new ListNode(0);
15// ListNode *cur = newHead;
16
17// while(a && b){
18// if(a->val <= b.val){
19// cur->val = a->val;
20// a = a->next;
21// }else
22// cur->val = b->val;
23// b = b->next;
24// }
25// }
26
27// if(a){
28// cur->next = a;
29// }else if(b){
30// cur->next = b;
31// }
32
33// return newHead;
34// };
35
36 ListNode* sortList(ListNode* head) {
37 if(!head) return head;
38 vector<int> arr;
39 ListNode* cur = head;
40
41 while(cur){
42 arr.push_back(cur->val);
43 cur = cur->next;
44 }
45
46 sort(arr.begin(), arr.end());
47
48 cur = head;
49 int i = 0;
50 while(cur){
51 cur->val = arr[i++];
52 cur = cur->next;
53 }
54
55 return head;
56 }
57};
58
59//merge sort
60//https://leetcode.com/problems/sort-list/discuss/46714/Java-merge-sort-solution
61//use new
62//Runtime: 124 ms, faster than 16.37% of C++ online submissions for Sort List.
63//Memory Usage: 48.8 MB, less than 5.00% of C++ online submissions for Sort List.
64//don't use new to speed up
65//Runtime: 72 ms, faster than 26.62% of C++ online submissions for Sort List.
66//Memory Usage: 26.7 MB, less than 5.00% of C++ online submissions for Sort List.
67/**
68 * Definition for singly-linked list.
69 * struct ListNode {
70 * int val;
71 * ListNode *next;
72 * ListNode(int x) : val(x), next(NULL) {}
73 * };
74 */
75class Solution {
76public:
77 ListNode* merge(ListNode* a, ListNode* b){
78 ListNode *newHead = new ListNode(0);
79 ListNode *cur = newHead;
80
81 // cout << "in merge a: " << endl;
82 cur = a;
83 while(cur){
84 // cout << cur->val << " ";
85 cur = cur->next;
86 }
87 // cout << endl;
88
89 // cout << "in merge b: " << endl;
90 // cur = b;
91 // while(cur){
92 // cout << cur->val << " ";
93 // cur = cur->next;
94 // }
95 // cout << endl;
96
97 cur = newHead;
98
99 // cout << "building list: " << endl;
100 while(a && b){
101 if(a->val <= b->val){
102 //use new -> time and space specified in first two rows
103 // cur->next = new ListNode(a->val);
104 //not use new -> time and space specified in second two rows
105 cur->next = a;
106 // cout << a->val << " ";
107 a = a->next;
108 }else{
109 // cur->next = new ListNode(b->val);
110 cur->next = b;
111 // cout << b->val << " ";
112 b = b->next;
113 }
114 cur = cur->next;
115 }
116
117 //Method 1 to add remaining list
118// while(a){
119// cur->next = new ListNode(a->val);
120// cout << a->val << " ";
121// a = a->next;
122// cur = cur->next;
123// }
124
125// while(b){
126// cur->next = new ListNode(b->val);
127// cout << b->val << " ";
128// b = b->next;
129// cur = cur->next;
130// }
131
132 //Method 2 to add remaining list
133 if(a){
134 cur->next = a;
135 }else if(b){
136 cur->next = b;
137 }
138
139 // cout << endl;
140
141 // cout << "list built: " << endl;
142 // cur = newHead->next;
143 // while(cur){
144 // cout << cur->val << " ";
145 // cur = cur->next;
146 // }
147 // cout << endl;
148
149 return newHead->next;
150 };
151
152 ListNode* sortList(ListNode* head) {
153 //length 0 or length 1, we cannot split anymore
154 if(!head || !head->next) return head;
155
156 // cout << "current list: " << endl;
157 ListNode* cur = head;
158 while(cur){
159 // cout << cur->val << " ";
160 cur = cur->next;
161 }
162 // cout << endl;
163
164 //split list
165 ListNode *slow = head, *fast = head;
166 ListNode *slowPrev; //used to cut the list into half
167
168 while(fast && fast->next){
169 slowPrev = slow;
170
171 slow = slow->next;
172 fast = fast->next->next;
173 }
174
175 /*
176 now "head" and "slow" are the head of two lists,
177 we need to split them by setting "slow"'s previous node's next to nullptr
178 */
179 slowPrev->next = nullptr;
180
181 //sort the splitted lists
182 head = sortList(head);
183 slow = sortList(slow);
184
185 return merge(head, slow);
186 }
187};Cost
Complexity
Time
O(n) to O(n log n), depending on the dominant loop or data structure operation
Dominated by the main traversal, recursion, or data-structure operations in the code.
Space
O(n) in the usual case for auxiliary containers or recursion
Auxiliary state plus the answer structure where the problem requires one.