Add Solution for 024 Swap Nodes in Pairs

cpp/024_Swap_Nodes_in_Pairs.cpp

@@ -0,0 +1,100 @@
+// 24. Swap Nodes in Pairs
+/**
+ * Given a linked list, swap every two adjacent nodes and return its head.
+ * 
+ * For example,
+ * Given 1->2->3->4, you should return the list as 2->1->4->3.
+ * 
+ * Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
+ * 
+ * Tags: Linked List
+ * 
+ * Similar Problems: (H) Reverse Nodes in k-Group
+ * 
+ * Author: Kuang Qin
+ */
+
+#include <iostream>
+
+using namespace std;
+
+/**
+ * Definition for singly-linked list.
+ */
+struct ListNode {
+    int val;
+    ListNode *next;
+    ListNode(int x) : val(x), next(NULL) {}
+    ListNode(int x, ListNode *p) : val(x), next(p) {}
+};
+
+// recursive solution
+class Solution {
+public:
+    ListNode* swapPairs(ListNode* head) {
+        if (head == NULL || head->next == NULL) {
+            return head;
+        }
+
+        ListNode *next = head->next;
+        head->next = swapPairs(next->next);
+        next->next = head;
+
+        return next;
+    }
+};
+
+// dummy pointer
+class Solution_Dummy {
+public:
+    ListNode* swapPairs(ListNode* head) {
+        ListNode dummy(0), *pre = &dummy;
+        pre->next = head;
+
+        while (pre->next != NULL && pre->next->next != NULL) {
+            // start: dummy->a->b->c
+            ListNode *curr = pre->next, *next = curr->next;     // pre: dummy, curr: a, next: b
+            curr->next = next->next;                            // point a->next to c
+            next->next = curr;                                  // point b->next to a
+            pre->next = next;                                   // point dummy->next to b
+            pre = curr;                                         // pre: a, go to next pair
+            // finish: dummy->b->a->c
+        }
+
+        return dummy.next;
+    }
+};
+
+// pointer to pointer
+class Solution_PP {
+public:
+    ListNode* swapPairs(ListNode* head) {
+        ListNode **pp = &head, *curr = head;
+
+        while (curr != NULL && curr->next != NULL) {
+            // start: head->a->b->c
+            ListNode *next = curr->next;    // curr: a, next: b, pp: &head
+            curr->next = next->next;        // point a->next to c
+            next->next = curr;              // point b->next to a
+            *pp = next;                     // point head to b
+            pp = &(curr->next);             // pp: &(a->next), go to next pair
+            curr = *pp;                     // curr: c
+            // finish: head->b->a->c
+        }
+
+        return head;
+    }
+};
+
+int main() {
+    ListNode node5(5), node4(4, &node5), node3(3, &node4), node2(2, &node3), node1(1, &node2);
+    Solution sol;
+    ListNode *newhead = sol.swapPairs(&node1);
+    for (ListNode *p = newhead; p != NULL; p = p->next) {
+        cout << p->val << " ";
+    }
+    cout << endl;
+    cin.get();
+
+    return 0;
+}