这里需要做一个假设：对于数组中下标为i的节点其左子树和右子树都是保持最大堆性质的堆。在假设成立的前提上，经过这一个维护函数维护过的堆才能够保证是一个最大堆。(There is a hypothesis that the left subtree and the right subtree of the node with the subscr ipt of I in the array are the heaps that maintain the maximum heap properties. On the premise of assumption, the heap that has been maintained through this maintenance function can be guaranteed to be the largest heap.)