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剑指 Offer II 026. 重排链表

题目描述

给定一个单链表 L 的头节点 head ，单链表 L 表示为：

 L0 → L1 → … → Ln-1 → Ln 
请将其重新排列后变为：

L0 → Ln → L1 → Ln-1 → L2 → Ln-2 → …

不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。

 

示例 1:

输入: head = [1,2,3,4]
输出: [1,4,2,3]

示例 2:

输入: head = [1,2,3,4,5]
输出: [1,5,2,4,3]

 

提示：

  • 链表的长度范围为 [1, 5 * 104]
  • 1 <= node.val <= 1000

 

注意：本题与主站 143 题相同：https://leetcode.cn/problems/reorder-list/ 

解法

方法一

Python3

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reorderList(self, head: ListNode) -> None:
        mid = self.middleNode(head)
        tmp = mid.next
        mid.next = None
        tmp = self.reverseList(tmp)
        head = self.mergeTwoLists(head, tmp)

    def middleNode(self, head: ListNode) -> ListNode:
        slow, fast = head, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        return slow

    def reverseList(self, head: ListNode) -> ListNode:
        pre, cur = None, head
        while cur:
            tmp = cur.next
            cur.next = pre
            pre = cur
            cur = tmp
        return pre

    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        dummy = ListNode()
        cur = dummy
        while l1 and l2:
            cur.next = l1
            l1 = l1.next
            cur = cur.next
            cur.next = l2
            l2 = l2.next
            cur = cur.next
        cur.next = l1 or l2
        return dummy.next

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        ListNode mid = middleNode(head);
        ListNode tmp = mid.next;
        mid.next = null;
        tmp = reverseList(tmp);
        head = mergeTwoLists(head, tmp);
    }

    private ListNode middleNode(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    private ListNode reverseList(ListNode head) {
        ListNode pre = null, cur = head;
        while (cur != null) {
            ListNode tmp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }

    private ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode();
        ListNode cur = dummy;
        while (l1 != null && l2 != null) {
            cur.next = l1;
            l1 = l1.next;
            cur = cur.next;
            cur.next = l2;
            l2 = l2.next;
            cur = cur.next;
        }
        cur.next = l1 != null ? l1 : l2;
        return dummy.next;
    }
}

C++

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode* head) {
        ListNode* mid = middleNode(head);
        ListNode* tmp = mid->next;
        mid->next = nullptr;
        tmp = reverseList(tmp);
        head = mergeTwoLists(head, tmp);
    }

    ListNode* middleNode(ListNode* head) {
        ListNode* slow = head;
        ListNode* fast = head;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }

    ListNode* reverseList(ListNode* head) {
        ListNode* pre = nullptr;
        ListNode* cur = head;
        while (cur) {
            ListNode* tmp = cur->next;
            cur->next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }

    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* dummy = new ListNode();
        ListNode* cur = dummy;
        while (l1 && l2) {
            cur->next = l1;
            l1 = l1->next;
            cur = cur->next;
            cur->next = l2;
            l2 = l2->next;
            cur = cur->next;
        }
        cur->next = l1 ? l1 : l2;
        return dummy->next;
    }
};

Go

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func reorderList(head *ListNode) {
	mid := middleNode(head)
	tmp := mid.Next
	mid.Next = nil
	tmp = reverseList(tmp)
	head = mergeTwoLists(head, tmp)
}

func middleNode(head *ListNode) *ListNode {
	slow, fast := head, head
	for fast != nil && fast.Next != nil {
		slow = slow.Next
		fast = fast.Next.Next
	}
	return slow
}

func reverseList(head *ListNode) *ListNode {
	var pre *ListNode
	cur := head
	for cur != nil {
		tmp := cur.Next
		cur.Next = pre
		pre = cur
		cur = tmp
	}
	return pre
}

func mergeTwoLists(l1 *ListNode, l2 *ListNode) *ListNode {
	dummy := new(ListNode)
	cur := dummy
	for l1 != nil && l2 != nil {
		cur.Next = l1
		l1 = l1.Next
		cur = cur.Next
		cur.Next = l2
		l2 = l2.Next
		cur = cur.Next
	}
	if l1 != nil {
		cur.Next = l1
	}
	if l2 != nil {
		cur.Next = l2
	}
	return dummy.Next
}

Swift

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     var val: Int
 *     var next: ListNode?
 *     init() { self.val = 0; self.next = nil; }
 *     init(_ val: Int) { self.val = val; self.next = nil; }
 *     init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
 * }
 */

class Solution {
    func reorderList(_ head: ListNode?) {
        guard let head = head else { return }

        let mid = middleNode(head)

        let secondHalf = reverseList(mid.next)
        mid.next = nil

        mergeTwoLists(head, secondHalf)
    }

    private func middleNode(_ head: ListNode?) -> ListNode {
        var slow = head
        var fast = head
        while fast != nil && fast?.next != nil {
            slow = slow?.next
            fast = fast?.next?.next
        }
        return slow!
    }

    private func reverseList(_ head: ListNode?) -> ListNode? {
        var prev: ListNode? = nil
        var curr = head
        while curr != nil {
            let nextTemp = curr?.next
            curr?.next = prev
            prev = curr
            curr = nextTemp
        }
        return prev
    }

    private func mergeTwoLists(_ l1: ListNode?, _ l2: ListNode?) {
        var l1 = l1
        var l2 = l2
        while l1 != nil && l2 != nil {
            let l1Next = l1?.next
            let l2Next = l2?.next

            l1?.next = l2
            if l1Next == nil {
                break
            }
            l2?.next = l1Next

            l1 = l1Next
            l2 = l2Next
        }
    }
}