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

一刷
题解：注意

     5
     / \
    5   5

算3棵树

用DFS的原理，和boolean变量保存它的左右是否为univalue subtree.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    private int count = 0;
   
    public int countUnivalSubtrees(TreeNode root) {
        helper(root);
        return count;
    }
    
    private boolean helper(TreeNode node){
        if(node == null) return true;
        boolean left = helper(node.left);
        boolean right = helper(node.right);
        if(left && right){
            if(node.left!=null && node.val!=node.left.val) return false;
            if(node.right!=null && node.val!= node.right.val) return false;
            count++;
            return true;
        }
        return false;
    }
}

二刷
recursion

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private int count = 0;
    
    public int countUnivalSubtrees(TreeNode root) {
        helper(root);
        return count;
    }
    
    private boolean helper(TreeNode root){
        if(root == null) return true;
        boolean left = helper(root.left);
        boolean right = helper(root.right);
        if(left && right){
            if(root.left!=null && root.val != root.left.val) return false;
            if(root.right!=null && root.val != root.right.val) return false;
            count++;
            return true;
        }
        return false;
    }
}