3 - Two Pointers Algorithm

228. Middle of Linked List

https://www.lintcode.com/problem/middle-of-linked-list/description?_from=ladder&&fromId=1

快慢指针，快指针每次走两步，慢指针每次一步。快指针到尾的时候，慢指针就是中点，时间复杂度为O(n / 2)

/**
 * 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 || head.next == null) return head;
        ListNode slow = head, fast = head.next;
        while(fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
}