Merge pull request neetcode-gh#21 from bryanhitc/brhitchc/cpp-143-reo…

…rder-list

[C++] 143: Reorder List

cpp/143-Reorder-List.cpp

@@ -0,0 +1,72 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ * int val;
+ * ListNode *next;
+ * ListNode() : val(0), next(nullptr) {}
+ * ListNode(int x) : val(x), next(nullptr) {}
+ * ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+ public:
+ void reorderList(ListNode* head) {
+ ListNode* prev = nullptr;
+ ListNode* slow = head;
+ ListNode* fast = head;
+
+ // set slow to middle node
+ while (fast != nullptr && fast->next != nullptr) {
+ prev = slow;
+ slow = slow->next;
+ fast = fast->next->next;
+ }
+
+ if (prev != nullptr) {
+ prev->next = reverseList(slow);
+ mergeTwoLists(head, prev->next);
+ }
+ }
+
+ private:
+ ListNode* reverseList(ListNode* head) {
+ ListNode* prev = nullptr;
+ ListNode* curr = head;
+
+ while (curr != nullptr) {
+ ListNode* next = curr->next;
+ curr->next = prev;
+ prev = curr;
+ curr = next;
+ }
+
+ return prev;
+ }
+
+ // invariant: list2 length = [list1 length, list1 length + 1]
+ ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
+ ListNode dummy;
+ ListNode* prev = &dummy;
+
+ ListNode* l1 = list1;
+ ListNode* l2 = list2;
+
+ while (l1 != list2) {
+ takeNode(prev, l1);
+ takeNode(prev, l2);
+ }
+
+ // handle odd number of nodes
+ if (l2 != nullptr) {
+ takeNode(prev, l2);
+ }
+
+ return dummy.next;
+ }
+
+ void takeNode(ListNode*& prev, ListNode*& curr) {
+ prev->next = curr;
+ prev = prev->next;
+ curr = curr->next;
+ }
+};