feat: add another js solution to lc problem: no.426

Author: tchx626

solution/0400-0499/0426.Convert Binary Search Tree to Sorted Doubly Linked List/Solution.js

@@ -27,3 +27,42 @@ var treeToDoublyList = function (root) {
   pre.right = head;
   return head;
 };
+
+//solution 2
+/**
+ * // Definition for a Node.
+ * function Node(val, left, right) {
+ *      this.val = val;
+ *      this.left = left;
+ *      this.right = right;
+ *  };
+ */
+
+/**
+ * @param {Node} root
+ * @return {Node}
+ */
+var treeToDoublyList = function (root) {
+    if (!root) return;
+    let stk = [];
+    let cur = root;
+    let pre, head;
+    while (cur || stk.length) {
+        while (cur) {
+            stk.push(cur);
+            cur = cur.left;
+        }
+        let top = stk.pop();
+        if (pre) {
+            pre.right = top;
+            top.left = pre;
+        } else {
+            head = top;
+        }
+        pre = top;
+        cur = top.right;
+    }
+    pre.right = head;
+    head.left = pre;
+    return head;
+};