83. 删除排序链表中的重复元素

迭代

class Solution {
    public ListNode deleteDuplicates(ListNode head) {

        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;

        ListNode cur = head;
        ListNode next;

        while (cur != null){

            next = cur.next;

            /**
             * 如果cur是最后一个节点，就结束循环了
             */
            if (next != null && cur.val == next.val){
                cur.next = next.next;
            }
			else {
                cur = cur.next;
            }
        }

        return dummyHead.next;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(1)
 */

递归

public class Algorithm {

    public static void main(String[] args) {

        int[] arr = {1, 2, 4, 4, 7, 8, 9, 9};
        ListNode head = new ListNode(arr);

        System.out.println(head);

        System.out.println(new Solution().deleteDuplicates(head));
    }
}

class Solution {
    public ListNode deleteDuplicates(ListNode head) {

        /**
         * 递归终止条件：遇到的节点为空
         */
        if (head == null){
            return head;
        }

        ListNode son = deleteDuplicates(head.next);

        /**
         * 如果子链表不为空，将子链表头节点和head进行对比，相同则返回son，否则将head加到链表首端
         */
        if(son != null && head.val == son.val){
            return son;
        }
        else {

            head.next = son;
            return head;
        }
    }
}

class ListNode{

    int val;
    ListNode next;
    ListNode() {}

    ListNode(int val) {

        this.val = val;

    }

    ListNode(int val, ListNode next) {

        this.val = val; this.next = next;
    }

    ListNode(int[] arr){

        if (arr == null || arr.length == 0){
            throw new IllegalArgumentException("数组是空的");
        }

        this.val = arr[0];
        ListNode prev = this;

        for (int i = 1; i < arr.length; i++) {

            prev.next = new ListNode(arr[i]);
            prev = prev.next;
        }
    }

    @Override
    public String toString(){

        StringBuilder str = new StringBuilder();

        ListNode curr = this;
        while (curr != null){
            str.append(curr.val + "——>");
            curr = curr.next;
        }
        str.append("null");

        return str.toString();
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(logn)
 */

https://leetcode-cn.com/problems/remove-duplicates-from-sorted-list/