82. 删除排序链表中的重复元素 II

递归法

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode p = head.next;
        if (head.val == p.val) {
            // 删除重复出现的数字
            while (p != null && head.val == p.val) {
                p = p.next;
            }
            head = deleteDuplicates(p);
        } else {
            head.next = deleteDuplicates(p);
        }
        return head;
    }
}

• 时间复杂度 O(n)
• 空间复杂度 O(n)

迭代法：双指针

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode p = dummy, q = head;
        while (q != null && q.next != null) {
            if (q.val == q.next.val) {
                while (q.next != null && q.val == q.next.val) {
                    q = q.next;
                }
                q = q.next;
                p.next = q;
            } else {
                p = q;
                q = q.next;
            }
        }
        return dummy.next;
    }
}

• 时间复杂度 O(n)
• 空间复杂度 O(1)