fd

Author: dingjikerbo

solution/src/main/java/com/inuker/solution/InsertionSortList.java

@@ -8,29 +8,22 @@
 
 public class InsertionSortList {
 
+    /**
+     * 其实很简单，就是从head中每次取一个节点，插到dummy中适当的位置
+     */
     public ListNode insertionSortList(ListNode head) {
-        if (head == null) {
-            return head;
-        }
-
-        ListNode helper = new ListNode(0); //new starter of the sorted list
-        ListNode cur = head; //the node will be inserted
-        ListNode pre = helper; //insert node between pre and pre.next
-        ListNode next = null; //the next node will be inserted
-        //not the end of input list
-        while (cur != null) {
-            next = cur.next;
-            //find the right place to insert
-            while (pre.next != null && pre.next.val < cur.val) {
-                pre = pre.next;
+        ListNode dummy = new ListNode(0);
+        for (ListNode next; head != null; head = next) {
+            next = head.next;
+            for (ListNode cur = dummy; cur != null; cur = cur.next) {
+                if (cur.next != null && head.val > cur.next.val) {
+                    continue;
+                }
+                head.next = cur.next;
+                cur.next = head;
+                break;
             }
-            //insert between pre and pre.next
-            cur.next = pre.next;
-            pre.next = cur;
-            pre = helper;
-            cur = next;
         }
-
-        return helper.next;
+        return dummy.next;
     }
 }

solution/src/main/java/com/inuker/solution/MaximumSubarray.java

@@ -6,6 +6,7 @@
 
 public class MaximumSubarray {
 
+    // dp[i]表示包含第i个元素时的最大和
     public int maxSubArray(int[] nums) {
         if (nums.length == 0) {
             return 0;

solution/src/main/java/com/inuker/solution/RecoverBinarySearchTree.java

@@ -19,6 +19,7 @@ public void recoverTree(TreeNode root) {
         second.val = val;
     }
 
+    // Morris遍历
     private void inorder(TreeNode root) {
         while (root != null) {
             if (root.left == null) {

solution/src/main/java/com/inuker/solution/SortList.java

@@ -6,8 +6,17 @@
  * Created by liwentian on 2017/9/11.
  */
 
+/**
+ * 这题采用merge sort，可自顶向下或自底向上
+ * 区别是自顶向下是递归的，需要额外的空间
+ * 而自底向上不用
+ * 时间复杂度都是O(nlgn)
+ */
 public class SortList {
 
+    /**
+     * 自顶向下
+     */
     public ListNode sortList(ListNode head) {
         if (head == null || head.next == null)
             return head;
@@ -27,26 +36,70 @@ public ListNode sortList(ListNode head) {
     }
 
     ListNode merge(ListNode l1, ListNode l2) {
-        ListNode l = new ListNode(0), p = l;
-
-        while (l1 != null && l2 != null) {
+        ListNode dummy = new ListNode(0), cur = dummy;
+        for ( ; l1 != null && l2 != null; ) {
             if (l1.val < l2.val) {
-                p.next = l1;
+                cur.next = l1;
                 l1 = l1.next;
-            } else {
-                p.next = l2;
+            }  else {
+                cur.next = l2;
                 l2 = l2.next;
             }
-            p = p.next;
+            cur = cur.next;
         }
+        cur.next = l1 != null ? l1 : l2;
+        return dummy.next;
+    }
 
-        if (l1 != null)
-            p.next = l1;
-
-        if (l2 != null)
-            p.next = l2;
+    /**
+     * 自底向上
+     */
+    public ListNode sortList2(ListNode head) {
+        int size = 0;
+        for (ListNode node = head; node != null; node = node.next, size++);
+        ListNode dummy = new ListNode(0);
+        ListNode left, right, cur, tail;
+        for (int step = 1; step <= size; step *= 2) {
+            for (cur = head, tail = dummy; cur != null; ) {
+                left = cur;
+                right = split(left, step);
+                cur = split(right, step);
+                tail = merge(left, right, tail);
+            }
+            head = dummy.next;
+        }
+        return dummy.next;
+    }
 
-        return l.next;
+    /**
+     * 将l1和l2合并后挂在tail下，返回新的tail
+     */
+    ListNode merge(ListNode l1, ListNode l2, ListNode tail) {
+        for ( ; l1 != null && l2 != null; ) {
+            if (l1.val < l2.val) {
+                tail.next = l1;
+                l1 = l1.next;
+            }  else {
+                tail.next = l2;
+                l2 = l2.next;
+            }
+            tail = tail.next;
+        }
+        tail.next = l1 != null ? l1 : l2;
+        for ( ; tail.next != null; tail = tail.next);
+        return tail;
     }
 
+    /**
+     * 从start开始取step个节点，返回下一个节点
+     */
+    private ListNode split(ListNode start, int step) {
+        for (step--; start != null && step > 0; step--, start = start.next);
+        if (start != null && start.next != null) {
+            ListNode next = start.next;
+            start.next = null;
+            return next;
+        }
+        return null;
+    }
 }

test/src/main/java/com/inuker/test/main.java

@@ -1,5 +1,6 @@
 package com.inuker.test;
 
+import com.leetcode.library.ListNode;
 import com.leetcode.library.TreeNode;
 
 import java.util.HashMap;
@@ -21,8 +22,23 @@
 public class main {
 
     public static void main(String[] args) {
-
     }
 
 
+    public ListNode insertionSortList(ListNode head) {
+        ListNode dummy = new ListNode(0);
+        for (ListNode next; head != null; head = next) {
+            next = head.next;
+            for (ListNode cur = dummy; cur != null; cur = cur.next) {
+                if (cur.next != null && head.val > cur.next.val) {
+                    continue;
+                }
+                head.next = cur.next;
+                cur.next = head;
+                break;
+            }
+        }
+        return dummy.next;
+    }
+
 }