Add Dart solution for Binary Tree Postorder Traversal

dart/binary_tree_postorder_traversal.dart

@@ -0,0 +1,31 @@
+/*
+Given the root of a binary tree, return the postorder traversal of its nodes' values.
+*/
+
+/*
+Definition for a binary tree node.
+class TreeNode {
+  int val;
+  TreeNode? left;
+  TreeNode? right;
+  TreeNode([this.val = 0, this.left, this.right]);
+}
+*/
+
+class Solution {
+  List<int> postorderTraversal(TreeNode? root) {
+    final List<int> ans = [];
+    postorder(root, ans);
+    return ans;
+  }
+
+  void postorder(TreeNode? root, List<int> ans) {
+    if (root == null) {
+      return;
+    }
+
+    postorder(root.left, ans);
+    postorder(root.right, ans);
+    ans.add(root.val);
+  }
+}