findPairWithGivenSumInADoublyLinkedList.cpp

Linked List/findPairWithGivenSumInDubleLinkedList.cpp

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+// C++ program to find a pair with given sum x.
+#include<bits/stdc++.h>
+using namespace std;
+
+// structure of node of doubly linked list
+struct Node
+{
+	int data;
+	struct Node *next, *prev;
+};
+
+// Function to find pair whose sum equal to given value x.
+void pairSum(struct Node *head, int x)
+{
+	// Set two pointers, first to the beginning of DLL
+	// and second to the end of DLL.
+	struct Node *first = head;
+	struct Node *second = head;
+	while (second->next != NULL)
+		second = second->next;
+
+	// To track if we find a pair or not
+	bool found = false;
+
+	// The loop terminates when two pointers
+	// cross each other (second->next
+	// == first), or they become same (first == second)
+	while (first != second && second->next != first)
+	{
+		// pair found
+		if ((first->data + second->data) == x)
+		{
+			found = true;
+			cout << "(" << first->data<< ", "
+				<< second->data << ")" << endl;
+
+			// move first in forward direction
+			first = first->next;
+
+			// move second in backward direction
+			second = second->prev;
+		}
+		else
+		{
+			if ((first->data + second->data) < x)
+				first = first->next;
+			else
+				second = second->prev;
+		}
+	}
+
+	// if pair is not present
+	if (found == false)
+		cout << "No pair found";
+}
+
+// A utility function to insert a new node at the
+// beginning of doubly linked list
+void insert(struct Node **head, int data)
+{
+	struct Node *temp = new Node;
+	temp->data = data;
+	temp->next = temp->prev = NULL;
+	if (!(*head))
+		(*head) = temp;
+	else
+	{
+		temp->next = *head;
+		(*head)->prev = temp;
+		(*head) = temp;
+	}
+}
+
+int main()
+{
+	struct Node *head = NULL;
+	insert(&head, 9);
+	insert(&head, 8);
+	insert(&head, 6);
+	insert(&head, 5);
+	insert(&head, 4);
+	insert(&head, 2);
+	insert(&head, 1);
+	int x = 7;
+
+	pairSum(head, x);
+
+	return 0;
+}