删除链表中重复的结点

题目：在一个排序的链表中，如何删除重复的结点？

解题思路

解决这个问题的第一步是确定删除的参数。当然这个函数需要输入待删除链表的头结点。头结点可能与后面的结点重复，也就是说头结点也可能被删除，所以在链表头添加一个结点。

接下来我们从头遍历整个链表。如果当前结点的值与下一个结点的值相同，那么它们就是重复的结点，都可以被删除。为了保证删除之后的链表仍然是相连的而没有中间断开，我们要把当前的前一个结点和后面值比当前结点的值要大的结点相连。我们要确保prev要始终与下一个没有重复的结点连接在一起。

结点定义

private static class ListNode {
    private int val;
    private ListNode next;
    public ListNode() {
    }
    public ListNode(int val) {
        this.val = val;
    }
    @Override
    public String toString() {
        return val + "";
    }
}

代码实现

public class Test57 {
    private static class ListNode {
        private int val;
        private ListNode next;
        public ListNode() {
        }
        public ListNode(int val) {
            this.val = val;
        }
        @Override
        public String toString() {
            return val + "";
        }
    }
    private static ListNode deleteDuplication(ListNode head) {
        // 为null
        if (head == null) {
            return null;
        }
//        // 只有一个结点
//        if (head.next == null) {
//            return head;
//        }
        // 临时的头结点
        ListNode root = new ListNode();
        root.next = head;
        // 记录前驱结点
        ListNode prev = root;
        // 记录当前处理的结点
        ListNode node = head;
        while (node != null && node.next != null) {
            // 有重复结点，与node值相同的结点都要删除
            if (node.val == node.next.val) {
                // 找到下一个不同值的节点，注意其有可能也是重复节点
                while (node.next != null && node.next.val == node.val) {
                    node = node.next;
                }
                // 指向下一个节点，prev.next也可能是重复结点
                // 所以prev不要移动到下一个结点
                prev.next = node.next;
            }
            // 相邻两个值不同，说明node不可删除，要保留
            else {
                prev.next = node;
                prev = prev.next;
            }
            node = node.next;
        }
        return root.next;
    }
    private static void print(ListNode head) {
        while (head != null) {
            System.out.print(head + "->");
            head = head.next;
        }
        System.out.println("null");
    }
    public static void main(String[] args) {
        test01();
        test02();
        test03();
        test04();
        test05();
        test06();
        test07();
        test08();
        test09();
        test10();
    }
    // 1->2->3->3->4->4->5
    private static void test01() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(2);
        ListNode n3 = new ListNode(3);
        ListNode n4 = new ListNode(3);
        ListNode n5 = new ListNode(4);
        ListNode n6 = new ListNode(4);
        ListNode n7 = new ListNode(5);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n7;
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // 1->2->3->4->5->6->7
    private static void test02() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(2);
        ListNode n3 = new ListNode(3);
        ListNode n4 = new ListNode(4);
        ListNode n5 = new ListNode(5);
        ListNode n6 = new ListNode(6);
        ListNode n7 = new ListNode(7);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n7;
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // 1->1->1->1->1->1->2
    private static void test03() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(1);
        ListNode n3 = new ListNode(1);
        ListNode n4 = new ListNode(1);
        ListNode n5 = new ListNode(1);
        ListNode n6 = new ListNode(1);
        ListNode n7 = new ListNode(2);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n7;
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // 1->1->1->1->1->1->1
    private static void test04() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(1);
        ListNode n3 = new ListNode(1);
        ListNode n4 = new ListNode(1);
        ListNode n5 = new ListNode(1);
        ListNode n6 = new ListNode(1);
        ListNode n7 = new ListNode(1);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n7;
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // 1->1->2->2->3->3->4->4
    private static void test05() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(1);
        ListNode n3 = new ListNode(2);
        ListNode n4 = new ListNode(2);
        ListNode n5 = new ListNode(3);
        ListNode n6 = new ListNode(3);
        ListNode n7 = new ListNode(4);
        ListNode n8 = new ListNode(4);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n7;
        n7.next = n8;
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // 1->1->2->3->3->4->5->5
    private static void test06() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(1);
        ListNode n3 = new ListNode(2);
        ListNode n4 = new ListNode(3);
        ListNode n5 = new ListNode(3);
        ListNode n6 = new ListNode(4);
        ListNode n7 = new ListNode(5);
        ListNode n8 = new ListNode(5);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n7;
        n7.next = n8;
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // 1->1->2->2->3->3->4->5->5
    private static void test07() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(1);
        ListNode n3 = new ListNode(2);
        ListNode n4 = new ListNode(2);
        ListNode n5 = new ListNode(3);
        ListNode n6 = new ListNode(3);
        ListNode n7 = new ListNode(4);
        ListNode n8 = new ListNode(5);
        ListNode n9 = new ListNode(5);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n7;
        n7.next = n8;
        n8.next = n9;
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // 1->2
    private static void test08() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(2);
        n1.next = n2;
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // 1
    private static void test09() {
        ListNode n1 = new ListNode(1);
        ListNode result = deleteDuplication(n1);
        print(result);
    }
    // null
    private static void test10() {
        ListNode result = deleteDuplication(null);
        print(result);
    }
}

运行结果