分治算法

build(arr, left, right):
    if left > right: return null
    mid = (left + right) / 2
    node = new Node(arr[mid])
    node.left  = build(arr, left, mid - 1)
    node.right = build(arr, mid + 1, right)
    return node

hanoi(n, from, aux, to):
    if n == 1: move disk 1 from → to; return
    hanoi(n-1, from, to, aux)   // 子问题：小盘先到辅助柱
    move disk n from → to
    hanoi(n-1, aux, from, to)