The hypothesis of this implementation is that size_type is unsigned.

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