116. 填充每个节点的下一个右侧节点指针

class Solution {
    public Node connect(Node root) {
        dfs(root);
        return root;
    }

    void dfs(Node root){
        if(root==null)  return;
        Node left = root.left;
        Node right = root.right;
        while(left!=null){
            left.next = right;
            left = left.right;
            right = right.left;
        }
        dfs(root.left);
        dfs(root.right);
    }
}