upt: edit 二叉树理论基础.md

添加JavaScript版的二叉树节点定义方式

Author: jwhx

problems/二叉树理论基础.md

@@ -223,6 +223,14 @@ type TreeNode struct {
 }
 ```
 
+JavaScript：
+```
+function TreeNode(val, left, right) {
+    this.val = (val===undefined ? 0 : val)
+    this.left = (left===undefined ? null : left)
+    this.right = (right===undefined ? null : right)
+}
+```
 
 
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