Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private int ans;

    public int countUnivalSubtrees(TreeNode root) {
        dfs(root);
        return ans;
    }

    private boolean dfs(TreeNode root) {
        if (root == null) {
            return true;
        }
        boolean l = dfs(root.left);
        boolean r = dfs(root.right);
        if (!l || !r) {
            return false;
        }
        int a = root.left == null ? root.val : root.left.val;
        int b = root.right == null ? root.val : root.right.val;
        if (a == b && b == root.val) {
            ++ans;
            return true;
        }
        return false;
    }
}