Leetcode, construct binary tree from inorder and post order traversal

Sept. 13, 2015

Spent more than a few hours to work on the leetcode problem, and my favorite blogs about this problems:

1. http://siddontang.gitbooks.io/leetcode-solution/content/tree/construct_binary_tree.html

2.http://blog.csdn.net/linhuanmars/article/details/24390157

After reading the above reference 1, Julia spent first few hours to write the C# implementation:

https://github.com/jianminchen/Leetcode_C-/blob/master/106ConstructuBTreeFromInorderPostOrderTraversal.cs

In her coding practice of function build(...), she spent over 20 minutes to figure out the coding task: the code to partition the inorder traversal into two partitions, first is left subtree, second is right subtree. And then, post order traversal also can be partitioned into two intervals, first one is for left subtree, and then, second one is for right subtree.

She took more than 10-15 minutes to understand the solution. That is too long for real problem solving. Figure out that only job is to find the root node, and then, its left child and right child is also the root node of left subtree/ or right subtree. The recursive call, actually two of them, can help to do the task.

So, she needs to cut down the practice time, and make sure the calculation is correct, easy to tell/ maintainable code/ testable code. So, she decided to practice it using class Range instead of two arguements start/ end integers. Hopefully, this practice will enhance her memory about partition the array into two parts, one is dividing in mid point, another one is by length of first interval - left subtree.

public class Range{
public int start, end;
...

}

Also, she likes to enhance her memory about this solution, so, she writes second implementation using C#, and see if the code can pass online judge. Most important, she tried to use different solution, to improve, challenge herself. Write the code without any mistake first time, in less than 10 minutes based on previous one.

https://github.com/jianminchen/Leetcode_C-/blob/master/106ConstructBTreeFromInOrderPostOrderTraversal_B.cs

Julia likes to train herself using leetcode questions, and biggest problem is to cut down the time to write a solution. She tries to cut down time from hours to 10-30 minutes.

After the code writing, she thought about more about great ideas out there, she should not miss. So, she reads the second reference, and like the most about the analysis:
"这道题和Construct Binary Tree from Preorder and Inorder Traversal是树中难度比较大的题目了，有朋友可能会想根据先序遍历和后序遍历能不能重新构造出树来，答案是否定的。只有中序便利可以根据根的位置切开左右子树，其他两种遍历都不能做到，其实先序遍历和后序遍历是不能唯一确定一棵树的，会有歧义发生，也就是两棵不同的树可以有相同的先序遍历和后序遍历，有兴趣的朋友可以试试举出这种例子."

Julia 发现在训练自己做题是, 如果能摸索出方法, 提高写代码的速度, 从几个小时, 到10-30 分钟, 那就是很成功的训练. 一种方式, 就是, 找到她喜欢的题解, 能够理解算法; 接下来, 看如何提高写代码的速度, 最好的方式, 就是多写几个解法, 看哪个不容易出错.

最后, 就是, 快速看十几个博客, 看有没有错过最重要, 最关键的分析.

接下来, 就是重复训练; 分析超时的原因, 能不能达到目的10-15分钟写出正确的代码. 就像网球训练, 训练自己.