CC4.8.java

    public boolean isSubtree(TreeNode t1, TreeNode t2) {
        if (t2 == null)
            return true;
        if (t1 == null)
            return false;
        if (isMatch(t1, t2))
            return true;
        return isSubtree(t1.left, t2) || isSubtree(t1.right, t2);
    }
    
    private boolean isMatch(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null)
            return true;
        if (t1 == null || t2 == null)
            return false;
        if (t1.val != t2.val)
            return false;
        return isMatch(t1.left, t2.left) && isMatch(t1.right, t2.right);
    }