链表相交

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目标

找到两个链表相交的起始节点，交点表示一个链表的结尾与另一个链表中的某个节点链接，形成Y形。如图所示：

具体实现

1. 计算出两个链表的长度：a_len和b_len
2. 再计算出两个链表长度的差：len­Diff = (a_len ~ b_len)
3. 通过lenDiff遍历较长的链表
4. 同时遍历这两个链表，判断是否有相同的节点node，如果有这个节点就返回，没有就返回null

时间复杂度O(n)，空间复杂度O(1)。

实现代码：

public class FindIntersectionOfLinkedLists {

	public LinkedListIntersection a;
	public LinkedListIntersection b;
	public void createLists(){
		a = new LinkedListIntersection();
			a.addAtEnd(1);
			a.addAtEnd(10);
			a.addAtEnd(20);
			Node tmp = a.addAtEnd(30);
			a.addAtEnd(40);
			a.addAtEnd(50);
			a.addAtEnd(60);
			System.out.print("List A : ");
			a.display();
		b = new LinkedListIntersection();
			b.addAtEnd(5);
			b.addAtEnd(15);
			b.createIntersection(a,tmp);
			System.out.print("List B : ");
			b.display();
	}
	public void findIntersectionByLength(){
		int a_len=0;
		int b_len=0;
		int lenDiff=0;
		boolean intsctFound = false;
		Node an = a.head;
		Node bn = b.head;
		//计算两个链表的长度：a_len和b_len
		while(an!=null){
			an=an.next;
			a_len++;
		}
		while(bn!=null){
			bn=bn.next;
			b_len++;
		}
		
		// 计算a、b两个链表的长度之差，用长链表减去短链表，根据差值遍历最长的链表，移动最长链表的节点指针，最终的结果会使两个链表长度相等
		an = a.head;
		bn = b.head;
		if(a_len>b_len){
			lenDiff = a_len-b_len;
		//	System.out.print("length diff " +lenDiff );
			while(lenDiff!=0){
				an = an.next;
				lenDiff--;
			}
		}else{
				lenDiff = b_len-a_len;
			while(lenDiff!=0){
				bn = bn.next;
				lenDiff--;
			}
		}
		
		// 同时遍历两个链表，找出交点
		while(an!=null && bn!=null){
			//System.out.print(an.data + " " + bn.data);
			if(an==bn) {
				System.out.print("交点是 " + an.data);
				intsctFound = true;
				break;
			}
			else{
				an = an.next;	
				bn = bn.next;
			}
		}
		if(!intsctFound){
			System.out.print("没有找到交点");
		}
	}
	
	public static void main (String[] args) throws java.lang.Exception
	{
		FindIntersectionOfLinkedLists i = new FindIntersectionOfLinkedLists();
		i.createLists();
		i.findIntersectionByLength();
	}
}
class Node{
	public int data;
	public Node next;
	public Node(int data){
		this.data = data;
		this.next = null;
	}
}
class LinkedListIntersection{
	public Node head;
	public LinkedListIntersection(){
		head=null;
	}
	
	public Node addAtEnd(int data){
		Node n = new Node(data);
		
		if (head==null){
			n.next = head;
			head = n;
		}
		else{
			Node currNode = head;
			while(currNode.next!=null){
				//System.out.print("---->" + currNode.data);
				currNode = currNode.next;
			}
			currNode.next = n;
		}
		return n;
	}
	public void createIntersection(LinkedListIntersection a, Node nd){
		Node hd = a.head; 
		while(hd!=nd){
			hd = hd.next;
		}
		Node currNode = head;
			while(currNode.next!=null){
				currNode = currNode.next;
			}
			currNode.next = hd; ;
	}
	public void display(){
		System.out.println("");
		Node currNode = head;
		while(currNode!=null){
			System.out.print("->" + currNode.data);
			currNode=currNode.next;
		}
		System.out.println("");
	}
}

编译自：Find Intersection Point in Two Linked List。