题目如下；

Quadtrees

A quadtree is a representation format used to encode images. The fundamental ideabehind the quadtree is that any image can be split into four quadrants. Each quadrant mayagain be split in four sub quadrants, etc. In the quadtree, the image is represented
by aparent node, while the four quadrants are represented by four child nodes, in a predeterminedorder.

Of course, if the whole image is a single color, it can be represented by a quadtree consistingof a single node. In general, a quadrant needs only to be subdivided if it consists of pixels ofdifferent colors. As a result, the quadtree need not be of uniform
depth.

A modern computer artist works with black-and-white images of units, for a totalof 1024 pixels per image. One of the operations
he performs is adding two images together,to form a new image. In the resulting image a pixel is black if it was black in at least one ofthe component images, otherwise it is white.

This particular artist believes in what he calls the preferred fullness:for an image to beinteresting (i.e. to sell for big bucks) the most important property is the number of filled(black) pixels in the image. So, before adding two images together,
he would like to knowhow many pixels will be black in the resulting image. Your job is to write a program that,given the quadtree representation of two images, calculates the number of pixels that areblack in the image, which is the result of adding the two
images together.

In the figure, the first example is shown (from top to bottom) as image, quadtree, pre-order string (defined below) and number of pixels. The quadrant numbering is shown at thetop of the figure.

Input Specification

The first line of input specifies the number of test cases (N) your program has to process.

The input for each test case is two strings, each string on its own line. The string is thepre-order representation of a quadtree, in which the letter ‘p‘ indicates a parent node, theletter ‘f‘ (full) a black quadrant and the letter ‘e‘
(empty) a white quadrant. It is guaranteedthat each string represents a valid quadtree, while the depth of the tree is not more than 5(because each pixel has only one color).

Output Specification

For each test case, print on one line the text ‘There are X black pixels.‘, where
X is the number of black pixels in the resulting image.

Example Input

3
ppeeefpffeefe
pefepeefe
peeef
peefe
peeef
peepefefe

Example Output

There are 640 black pixels.
There are 512 black pixels.
There are 384 black pixels.

这道题先建树，读到‘p’说明该建四个子树，读到‘f’或‘e’说明是叶子节点，把它的四个子树赋为空就可以了，然后同时遍历两棵树，如果遇到两棵树的对应的两节点中有一个是f，则total加上相应值，并将这两个节点的子树全部赋为空，因为接下来的子树就不需要再遍历了。如果两棵子树有一个为空，则将空的那颗建新树，并将其data值赋为e，四个子树赋为空，这样方便遍历且不影响total值，注意建树时是在复合结构中建树，所以应该在复合结构中遍历，若退出复合结构再统一遍历的话就会运行时错误，然后递归遍历两颗子树，当n>5时return。因为两颗子树中有一个为空有两种情况，应该用else if连接两种情况，防止重复遍历。

AC的代码如下：

UVA Quadtrees