update

Author: applewjg

ConvertSortedListtoBinarySearchTree.java

@@ -7,7 +7,9 @@
  Notes:
  Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
 
- Solution: Recursion. Pre-order. O(n)
+ Solution: 1. Recursion. In-order. O(n)
+           2. Recursion . Pre-order.
+           3. pre-order.
  */
 /**
  * Definition for singly-linked list.
@@ -27,10 +29,30 @@
  * }
  */
 public class Solution {
-    public TreeNode sortedListToBST(ListNode head) {
-        return sortedListToBSTRe(head, null);
+    public ListNode iter;
+    public TreeNode sortedListToBST_1(ListNode head) {
+        iter = head;
+        int len = 0;
+        while (head != null) {
+            ++len;
+            head = head.next;
+        }
+        return sortedListToBSTRe1(len);
+    }
+    public TreeNode sortedListToBSTRe1(int len) {
+        if (len == 0) return null;
+        int mid = len / 2;
+        TreeNode left = sortedListToBSTRe1(mid);
+        TreeNode root = new TreeNode(iter.val);
+        root.left = left;
+        iter = iter.next;
+        root.right = sortedListToBSTRe1(len - 1 - mid);
+        return root;
     }
-    public TreeNode sortedListToBSTRe(ListNode start, ListNode end) {
+    public TreeNode sortedListToBST_2(ListNode head) {
+        return sortedListToBSTRe2(head, null);
+    }
+    public TreeNode sortedListToBSTRe2(ListNode start, ListNode end) {
         if(start == end) return null;
         ListNode pre = null;
         ListNode slow = start;
@@ -40,11 +62,11 @@ public TreeNode sortedListToBSTRe(ListNode start, ListNode end) {
             slow = slow.next;
         }
         TreeNode node = new TreeNode(slow.val);
-        node.left = sortedListToBSTRe(start, slow);
-        node.right = sortedListToBSTRe(slow.next,end);
+        node.left = sortedListToBSTRe2(start, slow);
+        node.right = sortedListToBSTRe2(slow.next,end);
         return node;
     }
-    public TreeNode sortedListToBST_2(ListNode head) {
+    public TreeNode sortedListToBST_3(ListNode head) {
         if (head == null) return null;
         if (head.next==null) return new TreeNode(head.val);
         ListNode slow = head;
@@ -59,9 +81,9 @@ public TreeNode sortedListToBST_2(ListNode head) {
         TreeNode node = new TreeNode(slow.val);
         if(pre!=null) {
             pre.next = null;
-            node.left = sortedListToBST(head);
+            node.left = sortedListToBST_3(head);
         }
-        node.right = sortedListToBST(fast);
+        node.right = sortedListToBST_3(fast);
         return node;
     }
 }