中国大学MOOC-陈越、何钦铭-数据结构-2018春-03-树2 List Leaves

03-树2 List Leaves（25 分）

Given a tree, you are supposed to list all the leaves in the order of top down, and left to right.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (≤10) which is the total number of nodes in the tree -- and hence the nodes are numbered from 0 to N−1. Then N lines follow, each corresponds to a node, and gives the indices of the left and right children of the node. If the child does not exist, a "-" will be put at the position. Any pair of children are separated by a space.

Output Specification:

For each test case, print in one line all the leaves' indices in the order of top down, and left to right. There must be exactly one space between any adjacent numbers, and no extra space at the end of the line.

Sample Input:

8
1 -
- -
0 -
2 7
- -
- -
5 -
4 6

Sample Output:

4 1 5

思路：从上到下，从左至右，按层序遍历树，将叶子依次输出;

难点：将二维的树结构进行线性化;

队列实现：遍历从根节点开始,首先将根节点入队，然后开始执行循环：节点出队，访问该节点，其左右儿子入队;

参考：https://blog.youkuaiyun.com/shinanhualiu/article/details/49253499

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