24 Swap Nodes in Pairs

Author: int32bit

README.md

@@ -18,6 +18,7 @@
 + [21 Merge Two Sorted Lists(归并)](algorithms/MergeTwoSortedLists)
 + [22 Generate Parentheses(递归、搜索)](algorithms/GenerateParentheses)
 + [23 Merge k Sorted Lists(堆)](algorithms/MergekSortedLists)
++ [24 Swap Nodes in Pairs(链表操作)](algorithms/SwapNodesinPairs)
 + [26 Remove Duplicates from Sorted Array](algorithms/RemoveDuplicatesfromSortedArray)
 + [27 Remove Element](algorithms/RemoveElement)
 + [29 Divide Two Integers(位运算、最小负数绝对值、溢出检测)](algorithms/DivideTwoIntegers)

algorithms/SwapNodesinPairs/README.md

@@ -0,0 +1,50 @@
+## 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. 
+
+## Solution
+
+用三个指针p, q, pre, p指向当前节点，初始化为head，q指向p的下一个节点，pre指向上次完成的节点，初始化null
+
+交换p，q，同时需要修改pre的下一个节点(之前指向p，由于p和q交换位置，需要修改指向q):
+
+```cpp
+while (p && p->next) {
+	ListNode *q = p->next;
+	p->next = q->next;
+	q->next = p;
+	if (pre)
+		pre->next = q;
+	pre = p;
+	p = p->next;
+}
+```
+
+同时需要注意更新head指针。完整代码:
+
+```cpp
+ListNode *swapPairs(ListNode *head) {
+	if (head == nullptr || head->next == nullptr)
+		return head;
+	ListNode *p = head;
+	head = p->next;
+	ListNode *pre = nullptr;
+	while (p && p->next) {
+		ListNode *q = p->next;
+		p->next = q->next;
+		q->next = p;
+		if (pre)
+			pre->next = q;
+		pre = p;
+		p = p->next;
+	}
+	return head;
+}
+```

algorithms/SwapNodesinPairs/list.cpp

@@ -0,0 +1,88 @@
+#include <iostream>
+#include <cstdio>
+#include <vector>
+#include <algorithm>
+#include <string>
+using namespace std;
+#include <stdlib.h>
+struct ListNode {
+	int val;
+	ListNode *next;
+	ListNode(int x): val(x), next(nullptr) {}
+};
+class Solution {
+	public:
+		ListNode *swapPairs(ListNode *head) {
+			if (head == nullptr || head->next == nullptr)
+				return head;
+			ListNode *p = head;
+			head = p->next;
+			ListNode *pre = nullptr;
+			while (p && p->next) {
+				ListNode *q = p->next;
+				p->next = q->next;
+				q->next = p;
+				if (pre)
+					pre->next = q;
+				pre = p;
+				p = p->next;
+			}
+			return head;
+		}
+
+};
+int getLength(ListNode *head)
+{
+	int len = 0;
+	ListNode *p = head;
+	while (p) {
+		++len;
+		p = p->next;
+	}
+	return len;
+}
+void print(ListNode *head)
+{
+	if (head == nullptr) {
+		printf("NULL\n");
+		return;
+	}
+	struct ListNode *p = head;
+	while (p) {
+		printf("%d ", p->val);
+		p = p->next;
+	}
+	printf("\n");
+}
+ListNode * mk_list(ListNode **ha, int a[], int n)
+{
+	if (n < 1)
+		return nullptr;
+	ListNode *p = new ListNode(a[0]);
+	*ha = p;
+	for (int i = 1; i < n; ++i) {
+		ListNode *q = new ListNode(a[i]);
+		p->next = q;
+		p = q;
+	}
+	return p;
+}
+void free_list(struct ListNode *head)
+{
+	struct ListNode *p = head;
+	while (p) {
+		struct ListNode *q = p->next;
+		delete p;
+		p = q;
+	}
+}
+int main(int argc, char **argv)
+{
+	Solution solution;
+	struct ListNode *head = NULL;
+	int a[] = {1, 2};
+	mk_list(&head, a, 2);
+	head = solution.swapPairs(head);
+	print(head);
+	return 0;
+}

algorithms/Template/list/list.cpp

@@ -10,6 +10,10 @@ struct ListNode {
 	ListNode *next;
 	ListNode(int x): val(x), next(nullptr) {}
 };
+class Solution {
+	public:
+
+};
 int getLength(ListNode *head)
 {
 	int len = 0;