Create 0147-insertion-sort-list.cpp

Author: Abe0770

cpp/0147-insertion-sort-list.cpp

@@ -0,0 +1,69 @@
+/*
+  Given the head of a singly linked list, sort the list using insertion sort, and return the sorted list's head.
+  
+  The steps of the insertion sort algorithm:
+  
+  1. Insertion sort iterates, consuming one input element each repetition and growing a sorted output list.
+  2. At each iteration, insertion sort removes one element from the input data, finds the location it belongs within the sorted list and inserts it there.
+  3. It repeats until no input elements remain.
+
+  Ex. Input: head = [4,2,1,3]
+      Output: [1,2,3,4]
+  
+  Time : O(N^2)
+  Space: O(1)
+*/
+
+
+
+/**
+ * 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:
+    ListNode* insertionSortList(ListNode* head) {
+        if(head == NULL || head -> next == NULL) {
+            return head;
+        }
+
+        ListNode *ptr1 = head -> next, *sortedPtr = head;
+        while(ptr1 != NULL) {
+            if(ptr1 -> val < sortedPtr -> val) {
+                ListNode *ptr2 = head, *lagPtr = head;
+                while(true) {
+                    if(ptr2 -> val > ptr1 -> val) {
+                        if(ptr2 == head) {
+                            sortedPtr -> next = ptr1 -> next;
+                            ptr1 -> next = head;
+                            head = ptr1;
+                            ptr1 = sortedPtr -> next;
+                            break;
+                        }
+                        else {
+                            sortedPtr -> next = ptr1 -> next;
+                            ptr1 -> next = ptr2;
+                            lagPtr -> next = ptr1;
+                            ptr1 = sortedPtr -> next;
+                            break;
+                        }
+                    }
+                    lagPtr = ptr2;
+                    ptr2 = ptr2 -> next;
+                }
+                
+            } else {
+                sortedPtr = sortedPtr -> next;
+                ptr1 = ptr1 -> next;
+            }
+        }
+
+        return head;
+    }
+};