LC.250. Count Univalue Subtrees

https://leetcode.com/problems/count-univalue-subtrees/description/
Given a binary tree, count the number of uni-value subtrees.

A Uni-value subtree means all nodes of the subtree have the same value.

For example:
Given binary tree,
              5'''''
             / 
            1   5''''
           /    
          5' 5''  5'''

5 5 5    5
          
            5
return 4.

典型的后序遍历:从最下层比较 最后"打印" 的概念  (5,5,1) (#,5,5),(5) 从下面的叶子倒推树的结果

root = 5' and 5'' res = 2
root = 1 res = 2  返回FALSE
root = 5''' res = 3
root = 5'''' res= 4
root = 5''''' 因为 ROOT =1 返回FALSE 所以不需要看了 res = 4
time: o(n)
space: o(n)

另外，这道题如果 改成：
              5
             / 
            1   5
           /    
          3   5   5
答案也是 4: 3，5 都算"值一样的子树"

 1   // node value, count
 2     int res ;
 3     public int countUnivalSubtrees(TreeNode root) {
 4         res = 0;
 5         helper(root);
 6         return res ;
 7     }
 8 
 9     //根 叶子 都要 一样的值，才算一个SUBTREE
10     private boolean helper(TreeNode root) {
11         if (root ==null) return true ;
12         boolean leftCheck = helper(root.left) ;
13         boolean rightCheck = helper(root.right) ;
14         if (leftCheck && rightCheck) {
15             //如果叶子有值，则必须和当前根值一样; 两边叶子如果都没有值，则直接＋＋
16             if (root.left != null && root.left.val != root.val){
17                 return false;
18             }
19             if (root.right != null && root.right.val != root.val){
20                 return false;
21             }
22             res++ ;
23             return true;
24         }
25         return false;
26     }


重点题，需要反复体会