Count Univalue Subtrees
題意：
找出所有subtree值全一樣的個數。
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.
解題思路：
使用遞迴去作，不斷的把root的val傳下去作比較，一但比到錯的，則返回false。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int countUnivalSubtrees(TreeNode root) {
if (root == null) {
return 0;
}
int[] counter = new int[1];
count(root, counter, root.val);
return counter[0];
}
private boolean count(TreeNode root, int[] counter, int val) {
if (root == null) {
return true;
}
boolean left = count(root.left, counter, root.val);
boolean right = count(root.right, counter, root.val);
if (left && right) {
counter[0]++;
}
return left && right && root.val == val;
}
}