Leetcode 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:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

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.

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解题思路：

一定要正确理解题意。Intersection 就是两个链表最后有完全相同的地方。所以要从相同长度开始计算。开始我没有正确理解题意，怎么也想不出来。后来看了答案才明白，简单明了。

1. 得到2个链条的长度。

2. 将长的链条向前移动差值（len1 - len2）

3. 两个指针一起前进，遇到相同的即是交点，如果没找到，返回null.

相当直观的解法。空间复杂度O(1)， 时间复杂度O(m+n)

First calculate the length of two lists and find the difference. Then start from the longer list at the diff offset, iterate though 2 lists and find the node.

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Java code:

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        //First calculate the length of two lists and find the difference. 
        //Then start from the longer list at the diff offset, iterate though 2 lists and find the node.
        int len1 = 0;
        int len2 = 0;
        ListNode p1 = headA;
        ListNode p2 = headB;
        if(p1 == null || p2 == null) {
            return null;
        }
        
        while(p1!= null) {
            len1++;
            p1 = p1.next;
        }
         while(p2!= null) {
            len2++;
            p2 = p2.next;
        }
        
        int diff = 0;
        p1 = headA;
        p2 = headB;
        
        if(len1 > len2) {
            diff = len1 -len2;
            int i = 0;
            while(i< diff) {
                p1 = p1.next;
                i++;
            }
        }else {
            diff = len2- len1;
            int i = 0;
            while(i< diff) {
                p2 = p2.next;
                i++;
            }
        }
        
        while(p1 != null && p2 != null) {
            if(p1.val == p2.val) {
                return p1;
            }
            p1 = p1.next;
            p2 = p2.next;
        }
        return null;
    }

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Reference:

1. http://www.programcreek.com/2014/02/leetcode-intersection-of-two-linked-lists-java/

2. http://yucoding.blogspot.com/2014/12/leetcode-question-intersection-of-two.html