题目：

题解：

func connect(root *Node) *Node {
    start := root
    for start != nil {
        var nextStart, last *Node
        handle := func(cur *Node) {
            if cur == nil {
                return
            }
            if nextStart == nil {
                nextStart = cur
            }
            if last != nil {
                last.Next = cur
            }
            last = cur
        }
        for p := start; p != nil; p = p.Next {
            handle(p.Left)
            handle(p.Right)
        }
        start = nextStart
    }
    return root
}