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简单

面试题 02.06. 回文链表

English Version

题目描述

编写一个函数，检查输入的链表是否是回文的。

 

示例 1：

输入： 1->2
输出： false

示例 2：

输入： 1->2->2->1
输出： true

 

进阶：
你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

解法

方法一：快慢指针 + 反转链表

我们首先判断链表是否为空，如果为空，直接返回 true。

接下来，我们使用快慢指针找到链表的中点，如果链表长度为奇数，那么慢指针指向的就是中点，如果链表长度为偶数，那么慢指针指向的是中间两个节点的前一个节点。

然后，我们将慢指针之后的链表反转，这样我们就得到了链表的后半部分，其中链表头节点为 $p$。

最后，我们循环比较链表的前半部分和后半部分，如果有不相等的节点，直接返回 false，否则遍历完链表后返回 true。

时间复杂度 $O(n)$，其中 $n$ 为链表的长度。空间复杂度 $O(1)$。

Python3

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def isPalindrome(self, head: ListNode) -> bool:
        if head is None:
            return True
        slow, fast = head, head.next
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        p = slow.next
        slow.next = None
        dummy = ListNode()
        while p:
            next = p.next
            p.next = dummy.next
            dummy.next = p
            p = next
        p = dummy.next
        while p:
            if head.val != p.val:
                return False
            head = head.next
            p = p.next
        return True

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        if (head == null) {
            return true;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode p = slow.next;
        slow.next = null;
        ListNode dummy = new ListNode(0);
        while (p != null) {
            ListNode next = p.next;
            p.next = dummy.next;
            dummy.next = p;
            p = next;
        }
        p = dummy.next;
        while (p != null) {
            if (head.val != p.val) {
                return false;
            }
            head = head.next;
            p = p.next;
        }
        return true;
    }
}

C++

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        if (!head) {
            return true;
        }
        ListNode* slow = head;
        ListNode* fast = head->next;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode* p = slow->next;
        slow->next = nullptr;
        ListNode* dummy = new ListNode(0);
        while (p) {
            ListNode* next = p->next;
            p->next = dummy->next;
            dummy->next = p;
            p = next;
        }
        p = dummy->next;
        while (p) {
            if (head->val != p->val) {
                return false;
            }
            head = head->next;
            p = p->next;
        }
        return true;
    }
};

Go

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func isPalindrome(head *ListNode) bool {
	if head == nil {
		return true
	}
	slow, fast := head, head.Next
	for fast != nil && fast.Next != nil {
		slow, fast = slow.Next, fast.Next.Next
	}
	p := slow.Next
	slow.Next = nil
	dummy := &ListNode{}
	for p != nil {
		next := p.Next
		p.Next = dummy.Next
		dummy.Next = p
		p = next
	}
	p = dummy.Next
	for p != nil {
		if head.Val != p.Val {
			return false
		}
		head = head.Next
		p = p.Next
	}
	return true
}

TypeScript

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function isPalindrome(head: ListNode | null): boolean {
    if (!head) {
        return true;
    }
    let slow = head;
    let fast = head.next;
    while (fast && fast.next) {
        slow = slow.next;
        fast = fast.next.next;
    }
    let p = slow.next;
    slow.next = null;
    const dummy = new ListNode(0);
    while (p) {
        const next = p.next;
        p.next = dummy.next;
        dummy.next = p;
        p = next;
    }
    p = dummy.next;
    while (p) {
        if (head.val !== p.val) {
            return false;
        }
        head = head.next;
        p = p.next;
    }
    return true;
}

JavaScript

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {boolean}
 */
var isPalindrome = function (head) {
    if (!head) {
        return true;
    }
    let slow = head;
    let fast = head.next;
    while (fast && fast.next) {
        slow = slow.next;
        fast = fast.next.next;
    }
    let p = slow.next;
    slow.next = null;
    const dummy = new ListNode(0);
    while (p) {
        const next = p.next;
        p.next = dummy.next;
        dummy.next = p;
        p = next;
    }
    p = dummy.next;
    while (p) {
        if (head.val !== p.val) {
            return false;
        }
        head = head.next;
        p = p.next;
    }
    return true;
};

C#

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public bool IsPalindrome(ListNode head) {
        if (head == null) {
            return true;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode p = slow.next;
        slow.next = null;
        ListNode dummy = new ListNode(0);
        while (p != null) {
            ListNode next = p.next;
            p.next = dummy.next;
            dummy.next = p;
            p = next;
        }
        p = dummy.next;
        while (p != null) {
            if (head.val != p.val) {
                return false;
            }
            head = head.next;
            p = p.next;
        }
        return true;
    }
}

Swift

/**
* public class ListNode {
*    var val: Int
*    var next: ListNode?
*    init(_ x: Int) {
*        self.val = x
*        self.next = nil
*    }
* }
*/

class Solution {
    func isPalindrome(_ head: ListNode?) -> Bool {
        if head == nil {
            return true
        }

        var slow = head
        var fast = head?.next
        while fast != nil && fast?.next != nil {
            slow = slow?.next
            fast = fast?.next?.next
        }

        var p = slow?.next
        slow?.next = nil
        var dummy = ListNode(0)

        while p != nil {
            let next = p?.next
            p?.next = dummy.next
            dummy.next = p
            p = next
        }

        p = dummy.next
        var currentHead = head
        while p != nil {
            if currentHead?.val != p?.val {
                return false
            }
            currentHead = currentHead?.next
            p = p?.next
        }

        return true
    }
}