Merge pull request neetcode-gh#21 from bryanhitc/brhitchc/cpp-143-reorder-list

[C++] 143: Reorder List

Author: neetcode-gh

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;
+  }
+};