InvertBinaryTree.java

public TreeNode invertTree(TreeNode root) {
    return this.helper(root);
}

// post order
private TreeNode helper(TreeNode root) {
    if (root == null) {
        return root;
    }

    TreeNode l = helper(root.left);
    TreeNode r = helper(root.right);

    root.left = r;
    root.right = l;

    return root;
}

// Preorder
private TreeNode invertHelper(TreeNode root) {
    if (root == null) {
        return null;
    }

    TreeNode temp = root.left;
    root.left = root.right;
    root.right = temp;

    this.invertHelper(root.left);
    this.invertHelper(root.right);

    return root;
}