05：平衡树

算法原理：

PHP代码：

function isBalance($treenode, &$depth) {
    if ($treenode == null) {
        $depth = 0;
        return true;
    }
    if (isBalance($treenode->leftnode, $LH) && isBalance($treenode->rightnode, $RH)) {
        $diff = $LH - $RH;
        if ($diff <= 1 && $diff >= -1) {
            $depth = max($LH, $RH) + 1;
            return true;
        }
    }
    return false;
}