965. Univalued Binary Tree 

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class Solution {
    public boolean isUnivalTree(TreeNode root) {
        return dfs(root);
    }

    private boolean dfs(TreeNode root) {
        if (root == null) {
            return true;
        }
        if (root.left != null && root.left.val != root.val) {
            return false;
        }
        if (root.right != null && root.right.val != root.val) {
            return false;
        }

        return dfs(root.left) && dfs(root.right);
    }
}

References

965. Univalued Binary Tree