add 426

Author: fishercoder1534

Diff for: README.md

@@ -1019,6 +1019,7 @@ _If you like this project, please leave me a star._ ★
 | 432  | [All O`one Data Structure](https://leetcode.com/problems/all-oone-data-structure/)                                                                                                            | [Solution](../master/src/main/java/com/fishercoder/solutions/_432.java)                                                                  |                                                                               | Hard                             | Design                                                               
 | 430  | [Flatten a Multilevel Doubly Linked List](https://leetcode.com/problems/flatten-a-multilevel-doubly-linked-list/)                                                                             | [Solution](../master/src/main/java/com/fishercoder/solutions/_430.java)                                                                  |                                                                               | Medium                           | LinkedList, DFS, Doubly-Linked List                                  
 | 429  | [N-ary Tree Level Order Traversal](https://leetcode.com/problems/n-ary-tree-level-order-traversal/)                                                                                           | [Solution](../master/src/main/java/com/fishercoder/solutions/_429.java)                                                                  |                                                                               | Easy                             | BFS, Tree                                                            
+| 426  | [Convert Binary Search Tree to Sorted Doubly Linked List](https://leetcode.com/problems/convert-binary-search-tree-to-sorted-doubly-linked-list/)                                             | [Solution](../master/src/main/java/com/fishercoder/solutions/_426.java)                                                                  |                                                                               | Medium                           | DFS, BST, Recursion
 | 425  | [Word Squares](https://leetcode.com/problems/word-squares/)                                                                                                                                   | [Solution](../master/src/main/java/com/fishercoder/solutions/_425.java)                                                                  |                                                                               | Hard                             | Trie, Backtracking, Recursion                                        
 | 424  | [Longest Repeating Character Replacement](https://leetcode.com/problems/longest-repeating-character-replacement/)                                                                             | [Solution](../master/src/main/java/com/fishercoder/solutions/_424.java)                                                                  |                                                                               | Medium                           | Sliding Window                                                       
 | 423  | [Reconstruct Original Digits from English](https://leetcode.com/problems/reconstruct-original-digits-from-english/)                                                                           | [Solution](../master/src/main/java/com/fishercoder/solutions/_423.java)                                                                  |                                                                               | Medium                           | Math                                                                 

Diff for: src/main/java/com/fishercoder/solutions/_426.java

@@ -0,0 +1,54 @@
+package com.fishercoder.solutions;
+
+public class _426 {
+
+    public static class Solution1 {
+        Node head;
+        Node tail;
+
+        public Node treeToDoublyList(Node root) {
+            if (root == null) {
+                return null;
+            }
+            dfs(root);
+            tail.right = head;
+            head.left = tail;
+            return head;
+        }
+
+        private void dfs(Node node) {
+            if (node == null) {
+                return;
+            }
+            dfs(node.left);
+            if (head == null) {
+                head = node;
+            }
+            if (tail != null) {
+                tail.right = node;
+                node.left = tail;
+            }
+            tail = node;
+            dfs(node.right);
+        }
+    }
+
+    private static class Node {
+        public int val;
+        public Node left;
+        public Node right;
+
+        public Node() {
+        }
+
+        public Node(int _val) {
+            val = _val;
+        }
+
+        public Node(int _val, Node _left, Node _right) {
+            val = _val;
+            left = _left;
+            right = _right;
+        }
+    }
+}