binary tree 的inorder 和preorder 和postorder

In-order, pre-order, and post-order traversals are methods to visit each node in a binary tree in a specific sequence.

1. **In-order traversal**[^1]: Also known as the "left-root-right" order, it visits the left subtree first, then the root node, and finally the right subtree. This ensures that for a binary search tree, the output will be sorted.

   // Example for In-order traversal
   void inorderTraversal(TreeNode root) {
       if (root != null) {
           inorderTraversal(root.left);
           System.out.print(root.val + " "); // Visit the current node
           inorderTraversal(root.right);
       }
   }

2. **Pre-order traversal**: The "root-left-right" order, starts by visiting the root, followed by the left subtree, and then the right subtree.

   // Example for Pre-order traversal
   void preorderTraversal(TreeNode root) {
       if (root != null) {
           System.out.print(root.val + " "); // Visit the current node first
           preorderTraversal(root.left);
           preorderTraversal(root.right);
       }
   }

3. **Post-order traversal**: The "left-right-root" order, visits the left subtree, then the right subtree, and finally the root node.

   // Example for Post-order traversal
   void postorderTraversal(TreeNode root) {
       if (root != null) {
           postorderTraversal(root.left);
           postorderTraversal(root.right);
           System.out.print(root.val + " "); // Visit the current node last
       }
   }