Merge pull request #17 from zouyingjie/master

001 号第一周作业: LeetCode24-Swap Nodes in Pairs

Author: GeekUniversity

Week_01/id_1/LeetCode24.java

@@ -0,0 +1,44 @@
+class Solution {
+
+    ListNode prePairTail = null;
+
+    public ListNode swapPairs(ListNode head) {
+
+        if (head == null) {
+            return null;
+        }
+
+        if (head.next == null) {
+            return head;
+        }
+
+        ListNode result = head.next;
+        
+        ListNode node = head;
+        ListNode next = head.next;
+        while (node != null && next != null) {
+
+            node = swap(node, next);
+
+            if (node == null) {
+                break;
+            }
+            next = node.next;
+        }
+        return result;
+
+    }
+
+    private ListNode swap(ListNode node, ListNode next) {
+
+        ListNode tmpNext = next.next;
+        next.next = node;
+        node.next = tmpNext;
+        if (prePairTail != null) {
+            prePairTail.next = next;
+        }
+        prePairTail = node;
+        return tmpNext;
+    }
+
+}

Week_01/id_1/LeetCode25.java

@@ -0,0 +1,75 @@
+
+class Solution {
+
+    ListNode preGroupTail = null;
+    ListNode result = null;
+
+    public ListNode reverseKGroup(ListNode head, int k) {
+
+        if (head == null || head.next == null || k == 1) {
+            return head;
+        }
+
+        int index = 1;
+        ListNode groupHead = head;
+        ListNode groupTail = head;
+
+        while (true) {
+
+            if (index != k) { // 查找 k 个节点中的 head 和 tail 节点
+                groupTail = groupTail.next;
+                if (groupTail != null) {
+                    index ++;
+                    continue;
+                }else {
+                    // 链表长度小于 K 的情况
+                    if (this.preGroupTail != null) {
+                        this.preGroupTail.next = groupHead;
+                        break;
+                    }else {
+                        return head;
+                    }
+                }
+            }else { 
+                // 个数达到 k 时执行交换，返回下一组的 head 节点
+                ListNode node = this.swap(groupHead, groupTail);
+                if (node == null) {
+                    break;
+                }
+                groupHead = node;
+                groupTail = node;
+                index = 1;
+            }
+        }
+        return result;
+    }
+
+    private ListNode swap(ListNode head, ListNode tail) {
+
+        // 反转链表
+        ListNode preNode = null;
+        ListNode currentNode = head;
+        while (currentNode != tail) {
+            ListNode next = currentNode.next;
+            currentNode.next = preNode;
+            preNode = currentNode;
+            currentNode = next;
+        }
+
+        // 当前组的 tail 节点执行 swap
+        ListNode node= currentNode.next;
+        currentNode.next = preNode;
+
+        // 反转完成，将上一组的 tail 节点指向当前组的 head 节点
+        // 第一次反转时上一组 tail 为 null，因此要执行判空操作
+        if (this.preGroupTail != null) {
+            this.preGroupTail.next = currentNode;
+        }else {
+            // 上一组的 tail 为空说明是第一组执行交换
+            // 结果一定是第一组交换后的 head 节点
+            result = currentNode;
+        }
+        this.preGroupTail = head;
+        return node;
+    }
+}