LeetCode203 707链表

- 双向链表的使用
- 增加虚拟头节点方便操作

src/main/java/cn/whaifree/leetCode/middle/LeetCode707_2.java

@@ -0,0 +1,166 @@
+package cn.whaifree.leetCode.middle;
+
+import org.junit.Test;
+
+public class LeetCode707_2 {
+
+    @Test
+    public void test() {
+        MyLinkedList myLinkedList = new MyLinkedList();
+        myLinkedList.addAtHead(1);
+        myLinkedList.addAtHead(2);
+        myLinkedList.addAtHead(3);
+        myLinkedList.addAtHead(4);
+        myLinkedList.addAtHead(5);
+//        myLinkedList.addAtIndex(4, 99);
+        System.out.println(myLinkedList.get(0));
+        System.out.println(myLinkedList.get(1));
+        System.out.println(myLinkedList.get(2));
+        System.out.println(myLinkedList.get(3));
+        System.out.println(myLinkedList.get(4));
+        System.out.println(myLinkedList.get(-1));
+
+//        myLinkedList.deleteAtIndex(4);
+    }
+
+    class ListNode {
+        ListNode pre;
+        ListNode next;
+        int val;
+
+        public ListNode(ListNode pre, ListNode next, int val) {
+            this.pre = pre;
+            this.next = next;
+            this.val = val;
+        }
+    }
+
+
+    class MyLinkedList {
+        ListNode head;
+        ListNode tail;
+        int size;
+
+        public MyLinkedList() {
+            head = new ListNode(null, null, 0);
+            tail = new ListNode(head, null, 0);
+            head.next = tail;
+            size = 0;
+        }
+
+        public int get(int index) {
+            if (index >= size || index < 0) {
+                return -1;
+            }
+            if (index <= size / 2) {
+                ListNode trueIndex = head.next;
+                for (int i = 0; i < index; i++) {
+                    trueIndex = trueIndex.next;
+                }
+                return trueIndex.val;
+            } else {
+                ListNode trueIndex = tail.pre;
+                for (int i = size - 1; i > index; i--) {
+                    trueIndex = trueIndex.pre;
+                }
+                return trueIndex.val;
+            }
+        }
+
+        public void addAtHead(int val) {
+            ListNode listNode = new ListNode(head, head.next, val);
+            head.next.pre = listNode;
+            head.next = listNode;
+            size++;
+        }
+
+        public void addAtTail(int val) {
+            ListNode listNode = new ListNode(tail.pre, tail, val);
+            tail.pre.next = listNode;
+            tail.pre = listNode;
+            size++;
+        }
+
+        public void addAtIndex(int index, int val) {
+            if (index > size) {
+                return;
+            }
+
+            if (index == 0) {
+                addAtHead(val);
+                return;
+            }
+            if (index == size) {
+                addAtTail(val);
+                return;
+            }
+
+            if (index <= size / 2) {
+                ListNode trueIndex = head.next;
+                for (int i = 0; i < index; i++) {
+                    trueIndex = trueIndex.next;
+                }
+                ListNode listNode = new ListNode(trueIndex.pre, trueIndex, val);
+                trueIndex.pre.next = listNode;
+                trueIndex.pre = listNode;
+            } else {
+                ListNode trueIndex = tail.pre;
+                for (int i = size - 1; i > index; i--) {
+                    trueIndex = trueIndex.pre;
+                }
+                ListNode listNode = new ListNode(trueIndex.pre, trueIndex, val);
+                trueIndex.pre.next = listNode;
+                trueIndex.pre = listNode;
+            }
+            size++;
+        }
+
+        public void deleteAtIndex(int index) {
+            if (index >= size || index < 0) {
+                return;
+            }
+            if (index == 0) {
+                head.next.next.pre = head;
+                head.next = head.next.next;
+                size--;
+                return;
+            }
+            if (index == size - 1) {
+                tail.pre.pre.next = tail;
+                tail.pre = tail.pre.pre;
+                size--;
+                return;
+            }
+
+            if (index < size / 2) {
+                ListNode trueIndex = head.next;
+                for (int i = 0; i < index; i++) {
+                    trueIndex = trueIndex.next;
+                }
+                trueIndex.pre.next = trueIndex.next;
+                trueIndex.next.pre = trueIndex.pre;
+
+            } else {
+                ListNode trueIndex = tail.pre;
+                for (int i = size - 1; i > index; i--) {
+                    trueIndex = trueIndex.pre;
+                }
+                trueIndex.next.pre = trueIndex.pre;
+                trueIndex.pre.next = trueIndex.next;
+            }
+            size--;
+        }
+    }
+
+/**
+ * Your MyLinkedList object will be instantiated and called as such:
+ * MyLinkedList obj = new MyLinkedList();
+ * int param_1 = obj.get(index);
+ * obj.addAtHead(val);
+ * obj.addAtTail(val);
+ * obj.addAtIndex(index,val);
+ * obj.deleteAtIndex(index);
+ */
+}
+
+