250. Count Univalue Subtrees

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

return 

4

.
递归解题。如果root == null，返回true。如果左右都是true，而且root.val == root.left.val，root.val == root.right.val，count ++返回true，否则返回false。代码如下：
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
int count;

public int countUnivalSubtrees(TreeNode root) {
count = 0;
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;
}
}