背景

这是一个关于二叉树的一些算法题目，主要来源于 LeetCode

1. 判断同一颗树，100. Same Tree

    Input:    1         1
             / \       / \
            2   3     2   3
   
           [1,2,3],   [1,2,3]
   
    Output: true
   

2. 判断是否为子树，572. Subtree of Another Tree

   Given tree s:
   
          3
         / \
        4   5
       / \
      1   2
    Given tree t:
       4 
      / \
     1   2
    Return true, because t has the same structure and node values with a subtree of s.
   

3. 树的直径，树的两个节点的最大距离，543. Diameter of Binary Tree

          1
         / \
        2   3
       / \     
      4   5  
   
    Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].
    Note: The length of path between two nodes is represented 
          by the number of edges between them.
   

4. 树的路径和，找出所有路径和为给定值得总量 437. Path Sum III

   root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8
   
          10
         /  \
        5   -3
       / \    \
      3   2   11
     / \   \
    3  -2   1
    Return 3. The paths that sum to 8 are:
    1.  5 -> 3
    2.  5 -> 2 -> 1
    3. -3 -> 11