LinkedList - 160. Intersection of Two Linked Lists

160. Intersection of Two Linked Lists

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

begin to intersect at node c1.

Example 1:

```
Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,0,1,8,4,5], skipA = 2, skipB = 3
Output: Reference of the node with value = 8
Input Explanation: The intersected node's value is 8 (note that this must not be 0 if the two lists intersect). From the head of A, it reads as [4,1,8,4,5]. From the head of B, it reads as [5,0,1,8,4,5]. There are 2 nodes before the intersected node in A; There are 3 nodes before the intersected node in B.

思路：

题目意思是找出两个链表相交的节点，不相加返回null，这一题很直观的解法就是从较短的一个链表开始比较两个链表是否相交，这样做的前提是需要先求出两个链表的长度。这道题还可以把其中一个链表首尾相连，就变成在另一个链表找环那道题，因为是easy的题目，就用最直观的思路写一下。

代码：

java：

/**

 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        int lenA = length(headA), lenB = length(headB);
        
        while (lenA > lenB) {
            headA = headA.next;
            lenA--;
        }
        
        while (lenB > lenA) {
            headB = headB.next;
            lenB--;
        }
        
        while (headA != headB) {
            headA = headA.next;
            headB = headB.next;
        }
        
        return headA;
        
    }
    
    private int length(ListNode head) {
        int res = 0;
        while (head != null) {
            res++;
            head = head.next;
        }
        return res;
    }
}