思想：基于先序遍历，用一个静态变量保存WPL把每个节点的深度作为参数传递

若为叶子结点，WPL=该节点深度*权值，若非叶子节点则递归调用

代码：

typedef struct BTNode
{
    int weight;
    struct BTNode *lchild,*rchild;

}BTNode,*BTree;
int WPL(BTree root)
{
    return WPL_CORE(root,0);
}
int WPL_CORE(BTree root,int deep)
{
    static wpl=0;
    if(root->lchild==NULL&&root->rchild==null)
        wpl+=deep*root->weight;
    if(root->lchild!=NULL)
        WPL_CORE(root->lchild,deep+1);
    if(root->rchild!=NULL)
        WPL_CORE(root->lchild,deep+1);
    return wpl;

}