Checkpoint level 5

CheckpointProblems/(Level5)SUBTRACT.cpp

@@ -0,0 +1,77 @@
+/*
+Given linked list 1 -> 2 -> 3 -> 4 -> 5,
+
+Return 4 -> 2 -> 3 -> 4 -> 5
+as
+
+for first node, 5 - 1 = 4
+for second node, 4 - 2 = 2
+Try to solve the problem using constant extra space.
+https://www.interviewbit.com/problems/subtract/
+*/
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+
+void splitIntoTwo(struct ListNode *A, struct ListNode **f_ref, struct ListNode **b_ref) {
+	struct ListNode *slow, *fast;
+	slow = A;
+	fast = A->next;
+	while (fast != NULL) {
+		fast = fast->next;
+		if (fast != NULL) {
+			slow = slow->next;
+			fast = fast->next;
+		}
+	}
+	*f_ref = A;
+	*b_ref = slow->next;
+	slow->next = NULL;
+}
+
+void reverseList(struct ListNode **h_ref) {
+	struct ListNode *current, *prev, *next;
+	current = *h_ref;
+	prev = NULL;
+	while (current != NULL) {
+		next = current->next;
+		current->next = prev;
+		prev = current;
+		current = next;
+	}
+	*h_ref = prev;
+}
+
+void modifyFirstHalf(struct ListNode *front, struct ListNode *back) {
+	while (back != NULL) {
+		front->val = back->val - front->val;
+		front = front->next;
+		back = back->next;
+	}
+}
+struct ListNode *concatenate(struct ListNode *front, struct ListNode *back) {
+	struct ListNode *head = front;
+	while (front->next != NULL)
+		front = front->next;
+	front->next = back;
+	return head;
+}
+
+ListNode* Solution::subtract(ListNode* A) {
+	if ((!A) || (A->next == NULL))
+		return A;
+
+	struct ListNode *front, *back;
+	splitIntoTwo(A, &front, &back);
+	reverseList(&back);
+	modifyFirstHalf(front, back);
+	reverseList(&back);
+	A = concatenate(front, back);
+	return A;
+}