148 Sort List

Author: int32bit

README.md

@@ -86,6 +86,7 @@
 + [145 Binary Tree Postorder Traversal(后序遍历)](algorithms/BinaryTreePostorderTraversal)
 + [146 LRU Cache(空间换时间)](algorithms/LRUCache)
 + [147 Insertion Sort List(链表操作，插入排序)](algorithms/InsertionSortList)
++ [148 Sort List(归并排序)](algorithms/SortList)
 + [150 Evaluate Reverse Polish Notation(栈的应用)](algorithms/EvaluateReversePolishNotation)
 + [151 Reverse Words in a String](algorithms/ReverseWordsinaString)
 + [152 Maximum Product Subarray(DP)](algorithms/MaximumProductSubarray)

algorithms/SortList/README.md

@@ -0,0 +1,82 @@
+## Sort List
+
+Sort a linked list in O(n log n) time using constant space complexity.
+
+## Solution
+
+使用归并排序。
+
+首先需要把链表分成左右两部分，使用快慢指针法找到中间节点
+
+```cpp
+ListNode *slow = head;
+ListNode *fast = head,
+ListNode *prev = head; // 假设至少2个节点，因此while至少执行一次
+while (fast && fast->next) {
+	prev = slow;
+	slow = slow->next;
+	fast = fast->next->next;
+}
+prev->next = nullptr; // 断开左右部分
+```
+
+此时`left = head, right = slow`:
+
+```cpp
+ListNode *left = head;
+ListNode *right = slow;
+```
+
+分别调用mergeSort对左右部分排序:
+
+```cpp
+left = mergeSort(left);
+right = mergeSort(right);
+```
+
+然后合并左右两个部分即可:
+
+```cpp
+ListNode *merged = merge(left, right);
+```
+
+完整代码:
+
+```cpp
+ListNode *mergeSort(ListNode *head) {
+		if (!head || !head->next) // 少于2个节点直接返回
+			return head;
+		ListNode *slow = head;
+		ListNode *fast = head;
+		ListNode *prev = head; // 至少2个节点，因此while至少执行一次
+		while (fast && fast->next) {
+			prev = slow;
+			slow = slow->next;
+			fast = fast->next->next;
+		}
+		prev->next = nullptr;// 断开左右两部分
+		ListNode *left = mergeSort(head);
+		ListNode *right = mergeSort(slow);
+		ListNode *merged = merge(left, right);
+		return merged;
+	}
+```
+
+
+合并两个链表可以使用迭代法，也可以使用递归法，下面是递归实现方法:
+
+```cpp
+ListNode *merge(ListNode *l1, ListNode *l2) {
+	if (l1 == nullptr)
+		return l2;
+	if (l2 == nullptr)
+		return l1;
+	if (l1->val <= l2->val) {
+		l1->next = merge(l1->next, l2);
+		return l1;
+	} else {
+		l2->next = merge(l1, l2->next);
+		return l2;
+	}
+}
+```

algorithms/SortList/in.txt

@@ -0,0 +1,10 @@
+5
+5 4 3 2 1
+5
+1 2 3 5 4 
+5
+1 2 3 4 5
+1
+1
+5
+2 1 4 3 5

algorithms/SortList/list.cpp

@@ -0,0 +1,109 @@
+#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 *sortList(ListNode *head) {
+			return mergeSort(head);
+		}
+	private:
+		ListNode *mergeSort(ListNode *head) {
+			if (!head || !head->next)
+				return head;
+			ListNode *slow = head, *fast = head, *prev = head; // 至少2个节点，因此while至少执行一次
+			while (fast && fast->next) {
+				prev = slow;
+				slow = slow->next;
+				fast = fast->next->next;
+			}
+			prev->next = nullptr;
+			ListNode *left = mergeSort(head);
+			ListNode *right = mergeSort(slow);
+			ListNode *merged = merge(left, right);
+			return merged;
+		}
+		ListNode *merge(ListNode *l1, ListNode *l2) {
+			if (l1 == nullptr)
+				return l2;
+			if (l2 == nullptr)
+				return l1;
+			if (l1->val <= l2->val) {
+				l1->next = merge(l1->next, l2);
+				return l1;
+			} else {
+				l2->next = merge(l1, l2->next);
+				return l2;
+			}
+		}
+
+};
+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;
+	int a[20], n;
+	ListNode *head;
+	while (cin >> n) {
+		for (int i = 0; i < n; ++i)
+			cin >> a[i];
+		mk_list(&head, a, n);
+		print(head);
+		head = solution.sortList(head);
+		print(head);
+		cout << endl << endl;
+	}
+	return 0;
+}

algorithms/SortList/solve.cpp

@@ -0,0 +1 @@
+list.cpp