160. Intersection of Two Linked Lists(找出两个链表的交点) &&两个链表的第一个公共结点（剑指offer）

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.

Example 2:

Input: intersectVal = 2, listA = [0,9,1,2,4], listB = [3,2,4], skipA = 3, skipB = 1
Output: Reference of the node with value = 2
Input Explanation: The intersected node's value is 2 (note that this must not be 0 if the two lists intersect). From the head of A, it reads as [0,9,1,2,4]. From the head of B, it reads as [3,2,4]. There are 3 nodes before the intersected node in A; There are 1 node before the intersected node in B.

Example 3:

Input: intersectVal = 0, listA = [2,6,4], listB = [1,5], skipA = 3, skipB = 2
Output: null
Input Explanation: From the head of A, it reads as [2,6,4]. From the head of B, it reads as [1,5]. Since the two lists do not intersect, intersectVal must be 0, while skipA and skipB can be arbitrary values.
Explanation: The two lists do not intersect, so return null.

Notes:

• If the two linked lists have no intersection at all, return null.
• The linked lists must retain their original structure after the function returns.
• You may assume there are no cycles anywhere in the entire linked structure.
• Your code should preferably run in O(n) time and use only O(1) memory.

方法一：双指针

分析：链表a的长度a+c，链表b的长度：b+c 。链表重合部分的长度是：c

那么如果指向a的指针走过长度是：a+c+b

　　　　指向b的指针走过的长度是：b+c+a

的时候两个指针指向 的下一个结点就是第一个公共链的结点。（不然就是空节点，没有公共链）

时间复杂度：o（n+m）                               空间复杂度：o(1)

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        ListNode headA=pHead1;
        ListNode headB=pHead2;
        while (pHead1!=pHead2){
            pHead1=(pHead1==null)?headB:pHead1.next;
            pHead2=(pHead2==null)?headA:pHead2.next;
        }
        return pHead1;
    }
}

 方法二：利用hashset特性

hashset用于存储对象，用它存储pHead1链表所有结点。虽然它是无序的，不保证存入和取出的顺序相同，但是遍历pHead2的时候是有序的。遍历到第一个结点时就会立刻返回。

import java.util.HashSet;
/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        HashSet<ListNode> set=new HashSet<ListNode>();
        while (pHead1!=null){
            set.add(pHead1);
            pHead1=pHead1.next;
        }
        while (pHead2!=null){
            if(set.contains(pHead2))
                return pHead2;
            pHead2=pHead2.next;
        }
        return pHead2;
    }
}