Boundary Traversal of a Binary Tree

Understanding Boundary Traversal

class Solution {
    List<Integer> list;
    public List<Integer> boundaryOfBinaryTree(TreeNode root) {
        list= new ArrayList();
        list.add(root.val);
        leftview(root.left);
        leaves(root.left);
        leaves(root.right);
        rightview(root.right);
        return list;
    }
    public void leftview(TreeNode root){
        if(root==null) return;
        if(root.left==null&&root.right==null) return;
        this.list.add(root.val);
        if(root.left==null) leftview(root.right);
        else leftview(root.left);
    }
    public void leaves(TreeNode root){
        if(root==null) return;
        if(root.left==null&&root.right==null) this.list.add(root.val);
        leaves(root.left);
        leaves(root.right);
    }
    public void rightview(TreeNode root){
        if(root==null) return;
        if(root.left==null&&root.right==null) return;
        if(root.right==null) rightview(root.left);
        else rightview(root.right);
        this.list.add(root.val);
    }
}

Time Complexity : O(n)