update 426

Author: fishercoder1534

src/main/java/com/fishercoder/solutions/_426.java

@@ -3,6 +3,9 @@
 public class _426 {
 
     public static class Solution1 {
+        /**
+         * Credit: https://leetcode.com/problems/convert-binary-search-tree-to-sorted-doubly-linked-list/solutions/1494821/simplest-java-solution-with-explanation-inorder-traversal-in-place-no-dummy-node-needed/
+         */
         Node head;
         Node tail;
 
@@ -20,20 +23,25 @@ private void dfs(Node node) {
             if (node == null) {
                 return;
             }
+            //we traverse all the way to the most bottom left leaf node first
             dfs(node.left);
+            //we only need to assign head once, i.e. when it's not assigned, we'll assign the most bottom left leaf node to head
             if (head == null) {
                 head = node;
             }
+            //if the tail is already assigned, which should be all of the cases except when it's just finding the most bottom left leaf
+            // attach current node to tail's right, assign tail to current node's left
             if (tail != null) {
                 tail.right = node;
                 node.left = tail;
             }
+            //then always assign the current node to be the new tail
             tail = node;
             dfs(node.right);
         }
     }
 
-    private static class Node {
+    public static class Node {
         public int val;
         public Node left;
         public Node right;

src/test/java/com/fishercoder/_426Test.java

@@ -0,0 +1,30 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._426;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class _426Test {
+    private static _426.Solution1 solution1;
+
+    @BeforeEach
+    public void setup() {
+        solution1 = new _426.Solution1();
+    }
+
+    @Test
+    public void test1() {
+        _426.Node node1 = new _426.Node(1);
+        _426.Node node3 = new _426.Node(3);
+        _426.Node node5 = new _426.Node(5);
+        _426.Node node2 = new _426.Node(2);
+        node2.left = node1;
+        node2.right = node3;
+        _426.Node node4 = new _426.Node(4);
+        node4.left = node2;
+        node4.right = node5;
+        _426.Node actual = solution1.treeToDoublyList(node4);
+        System.out.println("Finished.");
+    }
+
+}