Symmetric Tree@leetcode

刷题必备书籍：Cracking the Coding Interview: 150 Programming Questions and Solutions 

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算法学习书籍：Introduction to Algorithms
编程珠玑：Programming Pearls (2nd Edition)
C++ 学习：The C++ Programming Language, 4th Edition
经典操作系统书籍，龙书：Operating System Concepts
创业：The Start-up of You: Adapt to the Future, Invest in Yourself, and Transform Your Career

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

Note:
Bonus points if you could solve it both recursively and iteratively.

这题递归法比较简单，几行代码的事情。但是我在oj的时候老是不通过，逻辑很简单，而且我确定是对的，后来自己跑到emacs上面调试，发现有segmentation fault，原来是我做判断的时候的一句话

if(l->val==r->val&&!l&&!r)

这种情况下，l or r 有可能是null，那么就没有l->val or r->val, 就会出现问题，这样把空判断提前就可以了，细节，但一定要注意！

这题难的是iterative的解法，基本做法是BFS，一层一层看是否对称，然后用两个queue来存储结点。