BoundaryOfABinaryTree.java

List<Integer> nodes = new ArrayList<>(1000);
public List<Integer> boundaryOfBinaryTree(TreeNode root) {
    
    if(root == null) return nodes;

    nodes.add(root.val);
    leftBoundary(root.left);
    leaves(root.left);
    leaves(root.right);
    rightBoundary(root.right);
    
    return nodes;
}
public void leftBoundary(TreeNode root) {
    if(root == null || (root.left == null && root.right == null)) return;
    nodes.add(root.val);
    if(root.left == null) leftBoundary(root.right);
    else leftBoundary(root.left);
}
public void rightBoundary(TreeNode root) {
    if(root == null || (root.right == null && root.left == null)) return;
    if(root.right == null)rightBoundary(root.left);
    else rightBoundary(root.right);
    nodes.add(root.val); // add after child visit(reverse)
}
public void leaves(TreeNode root) {
    if(root == null) return;
    if(root.left == null && root.right == null) {
        nodes.add(root.val);
        return;
    }
    leaves(root.left);
    leaves(root.right);
}