Preorder traversal of a binary tree (two ways)

1. traversal thought (root around)

Note: 1 each time the list will be empty;.

           2. The need to record the location on the back)
 

class Solution {
    List<Integer> list=new ArrayList<>();
    private void preorder(TreeNode root){
        if(root!=null){
        list.add(root.val);
        preorder(root.left);
        preorder(root.right);
        }
    }
    public List<Integer> preorderTraversal(TreeNode root) {
        list.clear();//清空
        preorder(root);
        return list;
    }
}

2. Summary of thought (the first one by traversing the left subtree, and then traverse the right subtree, during the last summary)

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        if(root==null)
        {
            return new ArrayList<>();//当root为空时，返回一个空链表
        }
        List<Integer> left=preorderTraversal(root.left);//求出所有左子树节点
        List<Integer> right=preorderTraversal(root.right);//求出右子树结点
        List<Integer> list=new ArrayList<>();
        list.add(root.val);
        list.addAll(left);//将链表节点全部进行尾插到新链表中
        list.addAll(right);
        return list;
    }
}