add 109

Author: shishishu

README.md

@@ -45,4 +45,5 @@ Idx | Date | Question | Python| Java | Domain | Tag | Difficulty | Remark
 039|20200425|[705. Design HashSet](https://leetcode.com/problems/design-hashset/)|[705p](https://github.com/shishishu/leetcode-python-java/blob/master/ipynb_files/705_Design_HashSet.ipynb)|[705j](https://github.com/shishishu/leetcode-python-java/blob/master/java_codes/705/Solution.java)|Hash| |Easy
 040|20200425|[13. Roman to Integer](https://leetcode.com/problems/roman-to-integer/)|[013p](https://github.com/shishishu/leetcode-python-java/blob/master/ipynb_files/013_Roman_to_Integer.ipynb)|[013j](https://github.com/shishishu/leetcode-python-java/blob/master/java_codes/013/Solution.java)|Math| |Easy
 041|20200425|[12. Integer to Roman](https://leetcode.com/problems/integer-to-roman/)|[012p](https://github.com/shishishu/leetcode-python-java/blob/master/ipynb_files/012_Integer_to_Roman.ipynb)|[012j](https://github.com/shishishu/leetcode-python-java/blob/master/java_codes/012/Solution.java)|Math| |Medium
-042|20200627|[108. Convert Sorted Array to Binary Search Tree](https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/)|[108p](https://github.com/shishishu/leetcode-python-java/blob/master/ipynb_files/108_Convert_Sorted_Array_to_Binary_Search_Tree.ipynb)|[108j](https://github.com/shishishu/leetcode-python-java/blob/master/java_codes/108/Solution.java)|BST|Recursive|Easy
+042|20200627|[108. Convert Sorted Array to Binary Search Tree](https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/)|[108p](https://github.com/shishishu/leetcode-python-java/blob/master/ipynb_files/108_Convert_Sorted_Array_to_Binary_Search_Tree.ipynb)|[108j](https://github.com/shishishu/leetcode-python-java/blob/master/java_codes/108/Solution.java)|BST|Recursive|Easy
+043|20200627|[109. Convert Sorted List to Binary Search Tree](https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/)|[109p](https://github.com/shishishu/leetcode-python-java/blob/master/ipynb_files/109_Convert_Sorted_List_to_Binary_Search_Tree.ipynb)|[109j](https://github.com/shishishu/leetcode-python-java/blob/master/java_codes/109/Solution.java)|BST|Recursive|Middle

images/109_Q.PNG

@@ -0,0 +1,88 @@
+import java.util.List;
+
+//Definition for singly-linked list.
+class ListNode {
+    int val;
+    ListNode next;
+    ListNode() {}
+    ListNode(int val) { this.val = val; }
+    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+}
+
+class TreeNode {
+    int val;
+    TreeNode left;
+    TreeNode right;
+    TreeNode() {}
+    TreeNode(int val) { this.val = val; }
+    TreeNode(int val, TreeNode left, TreeNode right) {
+        this.val = val;
+        this.left = left;
+        this.right = right;
+    }
+}
+
+// method 1: recursion
+class Solution {
+    public TreeNode sortedListToBST(ListNode head) {
+        if (head == null){
+            return null;
+        }
+        ListNode mid = findMiddle(head);
+        TreeNode node = new TreeNode(mid.val);
+        if (mid == head){
+            return node;
+        }
+        node.left = sortedListToBST(head);
+        node.right = sortedListToBST(mid.next);
+        return node;
+    }
+
+    private ListNode findMiddle(ListNode head){
+        ListNode prevPtr = null, slowPtr = head, fastPtr = head;
+        while ((fastPtr != null ) && (fastPtr.next != null)){
+            prevPtr = slowPtr;
+            slowPtr = slowPtr.next;
+            fastPtr = fastPtr.next.next;
+        }
+        if (prevPtr != null){
+            prevPtr.next = null;
+        }
+        return slowPtr;
+    }
+}
+
+// method 2: inorder simulation
+class Solution {
+
+    private ListNode head;
+
+    private int findSize(ListNode head){
+        ListNode ptr = head;
+        int c = 0;
+        while (ptr != null){
+            ptr = ptr.next;
+            c += 1;
+        }
+        return c;
+    }
+
+    private TreeNode convertListToBST(int l, int r){
+        if (l > r){
+            return null;
+        }
+        int mid = (l + r) / 2;
+        TreeNode left = this.convertListToBST(l, mid - 1);
+        TreeNode node = new TreeNode(this.head.val);
+        node.left = left;
+        this.head = this.head.next;
+        node.right = this.convertListToBST(mid + 1, r);
+        return node;
+    }
+
+    public TreeNode sortedListToBST(ListNode head) {
+        int size = findSize(head);
+        this.head = head;
+        return convertListToBST(0, size - 1);
+    }
+}

images/109_S1.PNG

@@ -46,6 +46,7 @@
       <sourceFolder url="file://$MODULE_DIR$/013" isTestSource="false" />
       <sourceFolder url="file://$MODULE_DIR$/012" isTestSource="false" />
       <sourceFolder url="file://$MODULE_DIR$/108" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/109" isTestSource="false" />
     </content>
     <orderEntry type="inheritedJdk" />
     <orderEntry type="sourceFolder" forTests="false" />