The trick here is to name the state correctly, then let the implementation follow. For 19. Remove Nth Node From End of List, the solution in this repository is mainly a straightforward implementation solution.
Guide
What?
We want to turn the problem statement into a smaller set of decisions the computer can repeat safely. 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: straightforward implementation.
The notes already sitting in the source point us in the right direction:
- linked list
Guide
When?
This pattern shows up when the brute force version has too many repeated checks, too many possible branches, or too much bookkeeping to do by hand. The accepted code reduces that pressure by storing exactly the information that remains useful later.
The important function names to track are removeNthFromEnd.
Guide
Why?
The solution works because it narrows the problem until every update has a clear reason to exist.
- 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:
- Read the setup variables first.
- Follow the main loop or recursive helper next.
- Watch where invalid states get skipped.
- Check which value survives to the return statement.
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(L), space: O(1)
- Space: O(n) in the usual case for auxiliary containers or recursion
Guide
C++ Solution
Your submission
The accepted solution
01//linked list
02//Runtime: 0 ms, faster than 100.00% of C++ online submissions for Remove Nth Node From End of List.
03//Memory Usage: 11 MB, less than 11.61% of C++ online submissions for Remove Nth Node From End of List.
04/**
05 * Definition for singly-linked list.
06 * struct ListNode {
07 * int val;
08 * ListNode *next;
09 * ListNode() : val(0), next(nullptr) {}
10 * ListNode(int x) : val(x), next(nullptr) {}
11 * ListNode(int x, ListNode *next) : val(x), next(next) {}
12 * };
13 */
14class Solution {
15public:
16 ListNode* removeNthFromEnd(ListNode* head, int n) {
17 ListNode* cur = head;
18 int len = 0;
19
20 while(cur){
21 ++len;
22 cur = cur->next;
23 }
24
25 if(n == len){
26 //delete the head
27 ListNode* ans = head->next;
28 delete head;
29 return ans;
30 }
31
32 cur = head;
33 for(int i = 0; i < len; ++i){
34 if(i == len-n-1){
35 //the node previous to the node to be deleted
36 ListNode* to_delete = cur->next;
37 cur->next = cur->next->next;
38 delete to_delete;
39 break;
40 }
41 cur = cur->next;
42 }
43
44 return head;
45 }
46};
47
48//Approach 1: Two pass algorithm
49//use dummy head to avoid corner case
50//Runtime: 8 ms, faster than 35.30% of C++ online submissions for Remove Nth Node From End of List.
51//Memory Usage: 10.9 MB, less than 18.24% of C++ online submissions for Remove Nth Node From End of List.
52//time: O(L), space: O(1)
53/**
54 * Definition for singly-linked list.
55 * struct ListNode {
56 * int val;
57 * ListNode *next;
58 * ListNode() : val(0), next(nullptr) {}
59 * ListNode(int x) : val(x), next(nullptr) {}
60 * ListNode(int x, ListNode *next) : val(x), next(next) {}
61 * };
62 */
63class Solution {
64public:
65 ListNode* removeNthFromEnd(ListNode* head, int n) {
66 ListNode* dummy = new ListNode();
67 dummy->next = head;
68
69 ListNode* cur;
70 int len = 0;
71
72 cur = dummy->next;
73 while(cur){
74 ++len;
75 cur = cur->next;
76 }
77
78 // cout << "len: " << len << endl;
79
80 cur = dummy;
81 for(int i = 0; i < len; ++i, cur = cur->next){
82 //i is 1-based
83 // cout << i << " : " << cur->val << endl;
84 if(i == len-n){
85 //this is the node previous to the node to be deleted
86 ListNode* tmp = cur->next;
87 cur->next = cur->next->next;
88 delete tmp;
89 break;
90 }
91 }
92
93 return dummy->next;
94 }
95};
96
97//Approach 2: One pass algorithm
98//Runtime: 4 ms, faster than 85.69% of C++ online submissions for Remove Nth Node From End of List.
99//Memory Usage: 11.1 MB, less than 5.24% of C++ online submissions for Remove Nth Node From End of List.
100//time: O(L), space: O(1)
101/**
102 * Definition for singly-linked list.
103 * struct ListNode {
104 * int val;
105 * ListNode *next;
106 * ListNode() : val(0), next(nullptr) {}
107 * ListNode(int x) : val(x), next(nullptr) {}
108 * ListNode(int x, ListNode *next) : val(x), next(next) {}
109 * };
110 */
111class Solution {
112public:
113 ListNode* removeNthFromEnd(ListNode* head, int n) {
114 ListNode* dummy = new ListNode();
115 dummy->next = head;
116
117 ListNode *slow = dummy, *fast = dummy;
118
119 for(int i = 0; i < n+1; ++i){
120 fast = fast->next;
121 }
122 //slow and fast are n+1 apart
123
124 while(fast){
125 slow = slow->next;
126 fast = fast->next;
127 }
128 //slow is n+1 from tail->next, so n from tail
129 //so it's (n+1)-last node
130
131 ListNode* tmp = slow->next;
132 slow->next = slow->next->next;
133 delete tmp;
134
135 return dummy->next;
136 }
137};Cost