回文链表 牛客网 程序员面试金典  C++ Python

• 题目描述
• 请编写一个函数，检查链表是否为回文。
• 给定一个链表ListNode* pHead，请返回一个bool，代表链表是否为回文。
• 测试样例：
• {1,2,3,2,1}
• 返回：true
• {1,2,3,2,3}
• 返回：false

class Palindrome {
public:
// run:4ms memory:492k
     bool isPalindrome(ListNode* pHead) {
        if(NULL == pHead) return true;
        if(NULL == pHead->next) return true;
        if(NULL == pHead->next->next)
            if(pHead->val == pHead->next->val) return true;
            else return false;
        bool ret = true;
        ListNode* lastNode = NULL;
        ListNode* nextNode = pHead->next;
        ListNode* pSlow = pHead;
        ListNode* pFast = pHead;
        while(pFast && pFast->next){
            pFast = pFast->next->next;
            pSlow->next = lastNode;
            lastNode = pSlow;
            pSlow = nextNode;
            nextNode = nextNode->next;
        }
        ListNode* lNode = lastNode;
        ListNode* nNode = pSlow;
        if (NULL == pFast)
            if (pSlow->val != lastNode->val) ret = false;
            else lastNode = lastNode->next;
        while(lastNode){
            if (lastNode->val != nextNode->val) {
                ret =  false;
                break;
            }

            lastNode = lastNode->next;
            nextNode = nextNode->next;
        }
        while(lNode){
            ListNode* tmp = lNode->next;
            lNode->next = nNode;
            nNode = lNode;
            lNode = tmp;
        }
        return ret;
    }

// run:5ms memory:600k
    bool isPalindrome2(ListNode* pHead) {
        if(NULL == pHead) return true;
        if(NULL == pHead->next) return true;
        if(NULL == pHead->next->next)
            if(pHead->val == pHead->next->val) return true;
            else return false;
        ListNode* lastNode = NULL;
        ListNode* nextNode = pHead->next;
        ListNode* pSlow = pHead;
        ListNode* pFast = pHead;
        while(pFast && pFast->next){
            pFast = pFast->next->next;
            pSlow->next = lastNode;
            lastNode = pSlow;
            pSlow = nextNode;
            nextNode = nextNode->next;
        }
        if (NULL == pFast)
            if (pSlow->val != lastNode->val) return false;
            else lastNode = lastNode->next;
        while(lastNode){
            if (lastNode->val != nextNode->val) return false;
            lastNode = lastNode->next;
            nextNode = nextNode->next;
        }
        return true;
    }

// run:4ms memory:476k
    bool isPalindrome3(ListNode* pHead) {
        if(NULL == pHead) return true;
        if(NULL == pHead->next) return true;
        if(NULL == pHead->next->next)
            if(pHead->val == pHead->next->val)
                return true;
            else return false;
        ListNode* lastNode = NULL;
        ListNode* nextNode = pHead->next;
        ListNode* pSlow = pHead;
        ListNode* pFast = pHead;
        while(pFast && pFast->next){
            pFast = pFast->next->next;
            pSlow->next = lastNode;
            lastNode = pSlow;
            pSlow = nextNode;
            nextNode = nextNode->next;
        }
        if (NULL == pFast){
            if (pSlow->val != lastNode->val)
                return false;
            else{
                lastNode = lastNode->next;
                while(lastNode){
                    if (lastNode->val != nextNode->val)
                        return false;
                    lastNode = lastNode->next;
                    nextNode = nextNode->next;
                }
            }
        }
        if(NULL == pFast->next){
               while(lastNode){
                    if (lastNode->val != nextNode->val)
                        return false;
                    lastNode = lastNode->next;
                    nextNode = nextNode->next;
                }
        }
        return true;
    }
};

Python

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Palindrome:
# run:43ms memory:5732k
    def isPalindrome(self, pHead):
        if None == pHead: return True
        if None == pHead.next: return True
        if None == pHead.next.next:
            if pHead.val == pHead.next.val: return True
            else: return False
        ret = True
        lastNode = None
        nextNode = pHead
        pFast = pHead
        pSlow = pHead
        while pFast and pFast.next:
            pFast = pFast.next.next
            nextNode = pSlow.next
            pSlow.next = lastNode
            lastNode = pSlow
            pSlow = nextNode
            nextNode = nextNode.next
        lNode = lastNode
        nNode = pSlow
        if None == pFast:
            if pSlow.val != lastNode.val:ret = False
            else:lastNode = lastNode.next
        while lastNode and nextNode:
            if lastNode.val != nextNode.val:
                ret = False
            lastNode = lastNode.next
            nextNode = nextNode.next
        while lNode:
            tmp = lNode.next
            lNode.next = nNode
            nNode = lNode
            lNode = tmp
        return ret

# run:34ms memory:5728k
    def isPalindrome2(self, pHead):
        if None == pHead: return True
        if None == pHead.next: return True
        if None == pHead.next.next:
            if pHead.val == pHead.next.val: return True
            else: return False
        lastNode = None
        nextNode = pHead
        pFast = pHead
        pSlow = pHead
        while pFast and pFast.next:
            pFast = pFast.next.next
            nextNode = pSlow.next
            pSlow.next = lastNode
            lastNode = pSlow
            pSlow = nextNode
            nextNode = nextNode.next
        if None == pFast:
            if pSlow.val != lastNode.val:return False
            else:lastNode = lastNode.next
        while lastNode:
            if lastNode.val != nextNode.val:return False
            lastNode = lastNode.next
            nextNode = nextNode.next
        return True