Implement strStr()

lintcode

Middle of Linked List Find the middle node of a linked list.

Example Example 1:

Input: 1->2->3 Output: 2 Explanation: return the value of the middle node. Example 2:

Input: 1->2 Output: 1 Explanation: If the length of list is even return the value of center left one. Challenge If the linked list is in a data stream, can you find the middle without iterating the linked list again?

思路

• 快慢指针,要想好奇数偶数问题
• 如果是判断 fast != null && fast.next != null 就解决了这个问题

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param head: the head of linked list.
     * @return: a middle node of the linked list
     */
    public ListNode middleNode(ListNode head) {
        // write your code here
        if (head == null) {
            return null;
        }

        ListNode dummy = new ListNode(0);
        dummy.next = head;

        ListNode slow = dummy;
        ListNode fast = dummy;

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }

        return slow;

    }
}