107. Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).

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For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
return its bottom-up level order traversal as:
[
  [15,7],
  [9,20],
  [3]
]

106. Construct Binary Tree from Inorder and Postorder Traversal

Given inorder and postorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.

105. Construct Binary Tree from Preorder and Inorder Traversal

Given preorder and inorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.

104. Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

103. Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes’ values. (ie, from left to right, then right to left for the next level and alternate between).

For example :
Given binary tree[3, 9, 20, null, null, 15, 7],

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 　 3
　 / \
  9  20
 / \
15  7
return its zigzag level order traversal as :
[
	[3],
	[20, 9],
	[15, 7]
]

C++ const 用法

　　关于指针，主要注意四个方面：指针类型、指针指向类型、指针的值、指针指向的值。当const修饰的是指针类型，那么指针的值就不能改变，即不能指向其他内存空间，但是可以通过指针修改其指向内存空间里面的值。当const修饰是指针指向的内存空间时，那么指针可以指向其他内存空间，但是不能通过指针修改指针所指向内存空间里面的值。

　　C++ const 允许指定一个语义约束，编译器会强制实施这个约束，允许程序员告诉编译器某值是保持不变的。如果在编程中确实有某个值保持不变，就应该明确使用const，这样可以获得编译器的帮助。

102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],
　　　3
　　/ 　\
　　9 　20
　 / \
　15 7

return its level order traversal as:
[
　[3],
　[9,20],
　[15,7]
]

101. Symmetric Tree

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

But the following [1,2,2,null,3,null,3] is not:

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

100. Same Tree

Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

99. Recover Binary Search Tree

Two elements of a binary search tree(BST) are swapped by mistake.
Recover the tree without changing its structure.

Note:
A solution using O(n) space is pretty straight forward.Could you devise a constant space solution ?