树的相关操作，先序遍历建立二叉链表，左右孩子交换，二叉树中叶子节点个数算法，统计以值为x的结点为根的子树中叶子结点的数目，按树状打印二叉树-Tree-related operations, first traversal establishment of binary list, so the child exchange, the number of leaf nodes in a binary tree algorithm, the statistical value of x to a n