重排链表——力扣143

题目描述

法一：寻找链表中点、链表逆序、链表合并

void reorderList(ListNode* head){
		if(head==nullptr){
			return;
		}
		// 找到中点 
		ListNode* mid = FindMiddle(head);
		ListNode *h1=head, *h2=mid->next;
		mid->next=nullptr;   
		// 逆序后半部分
		h2 = reverseList(h2);
		// 合并链表
		mergeList(h1, h2);
	} 
	
	ListNode* FindMiddle(ListNode* head){
		ListNode *slow=head, *fast=head;
		while(fast->next!=nullptr && fast->next->next!=nullptr){
			slow = slow->next;
			fast = fast->next->next;
		}
		return slow;
	}
	
	ListNode* reverseList(ListNode* head){
		ListNode* prev=nullptr;
		ListNode* cur = head;
		while(cur){
			ListNode* next = cur->next;
			cur->next = prev;
			prev = cur;
			cur = next;
		}
		return prev;
	}
	
	void mergeList(ListNode* l1, ListNode* l2){
		ListNode* l1_tmp;
		ListNode* l2_tmp;
		while(l1 && l2){
			l1_tmp = l1->next;
			l2_tmp = l2->next;
			
			l1->next = l2;
			l1 = l1_tmp;
			
			l2->next = l1;
			l2 = l2_tmp;
		}
	}