回文链表——力扣234

题目描述

法一 将值复制到数组中后用双指针法

class Solution {
public:
    bool isPalindrome(ListNode* head) {
        vector<int> v;
        while(head!=NULL){
            v.emplace_back(head->val);
            head = head->next;
        }
        for(int i=0, j=v.size()-1; i<j; i++, j--){
            if(v[i]!=v[j]){
                return false;
            }
        }
        return true;
    }
};

法二 快慢指针

class Solution{
public:
	bool isPalindrome(ListNode* head){
		if(head == NULL){
			return true;
		}
		
		ListNode* FirstHalfEnd = fun1(head);
		ListNode* SecondHalfStart = fun2(FirstHalfEnd->next);
		
		bool res = true;
		ListNode* p1 = head;
		ListNode* p2 = SecondHalfStart;
		while(res && p2!=NULL){
			if(p1->val != p2->val){
				res = false;
			}
			p1 = p1->next;
			p2 = p2->next;
		}
		FirstHalfEnd = fun2(SecondHalfStart)
		return res;
	}
	
	ListNode* fun1(ListNode* head){
		ListNode* fast = head;
		ListNode* slow = head;
		while(fast->next!=NULL && fast->next->next!=NULL){
			fast = fast->next->next;
			slow = slow->next;
		}
		return slow;
	}
	
	ListNode* fun2(ListNode* head){
		ListNode* prev = NULL;
		ListNode* cur = head;
		while(cur!=NULL){
			ListNode* temp = cur->next;
			cur->next = prev;
			prev = cur;
			cur = temp;
		}
		return prev;
	}
};