160. Intersection of Two Linked Lists

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

 Notice

• 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.

Example

The following two linked lists:

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

begin to intersect at node c1.

分析：

需要先得到两个List的长度，然后让长的List先走，等到长度一致的时候，比较两个指针是否指向同一个node，如果是，返回该node。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;      
 *     }
 * }
 */
public class Solution {
    /**
     * @param headA: the first list
     * @param headB: the second list
     * @return: a ListNode 
     * cnblogs.com/beiyeqingteng
     */
    public ListNode getIntersectionNode(ListNode h1, ListNode h2) {
        if (h1 == null || h2 == null) return null;
        
        int size1 = size(h1);
        int size2 = size(h2);
        
        for (int i = 0; i < Math.abs(size1 - size2); i++) {
            if (size1 > size2) {            
                h1 = h1.next;
            } else {
                h2 = h2.next;
            }    
        }
        
        while (h1 != h2) {
            h1 = h1.next;
            h2 = h2.next;
        }
        
        return h1;
    }
    
    private int size(ListNode head) {
        int total = 0;
        while (head != null) {
            total++;
            head = head.next;
        }
        return total;
    }
}

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