PairSwap.c

#include <stdio.h>
#include <assert.h>
#include <stdlib.h>

typedef struct Node {
		int data;
		struct Node *next;
		struct Node *prev;
} Node;

Node* SwapPairs(Node *head)
{
		//Except cases
		if (!head) return NULL;
		if (!head->next) return head;

		Node *toReturn = head->next;

		while(1) {
				Node *left = head->prev;
				Node *right = head->next;

				if (left) {
						//Head's left is currently incorrect...fix
						head->prev = head->prev->next;
						left = head->prev;
						left->next = right ? right : head;
				}

				if (right) {
						head->next = right->next;
						head->prev = right;
						right->next = head;
						right->prev = left;
				} 

				if (!head->next) break;
				head = head->next;
		};

		return toReturn;
}

Node* makeNode(int data, Node *parent)
{
		Node *n = malloc(sizeof (*n));
		n->data = data;
		n->prev = parent;
		n->next = NULL;
		if (parent) parent->next = n;

		return n;
}

void freeNodes(Node *head)
{
		Node *nxt = head ? head->next : NULL;
		free(head);
		if (nxt) freeNodes(nxt);
}

void printNodes(Node *head)
{
		if (head) printf("%d ", head->data);
		else {
				puts("");
				return;
		}

		printNodes(head->next);
}

int main()
{
		Node *n = NULL, *test;
		test = SwapPairs(n);
		assert(test == NULL);

		puts("Test 1");
		n = makeNode(1, NULL);
		n = SwapPairs(n);
		assert(n->data == 1);
		assert(n->next == NULL);

		puts("Test 2");
		makeNode(2, n);
		n = SwapPairs(n);
		assert(n->data == 2);
		assert(n->next->data == 1);

		puts("Test 3");
		makeNode(3, n->next);
		n = SwapPairs(n);
		assert(n->next->data == 2);
		assert(n->next->next->data == 3);

		puts("Test 4");
		makeNode(4, n->next->next);
		n = SwapPairs(n);
		assert(n->data == 2);
		assert(n->next->data == 1);
		assert(n->next->next->data == 4);
		assert(n->next->next->next->data == 3);
		assert(n->next->next->next->next == NULL);
		assert(n->next->next->next->prev == n->next->next);

		puts("All passed");
		freeNodes(n);
}