怎么让一个不定宽高的div，垂直水平居中

<div class="parent">
        <div class="son">123</div>
</div>

.parent{
            width: 200px;
            height: 200px;
            background-color: red;
            position: relative;
        }
        .son{
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%,-50%);
        }

.parent{
            width: 200px;
            height: 200px;
            background-color: red;
            display: flex;
            justify-content: center;
            align-items: center;
}

.parent{
            width: 200px;
            height: 200px;
            background-color: red;
            display:table-cell;
            text-align: center;
            vertical-align: middle;
        }
        .son{
            display: inline-block;
            vertical-align: middle;
        }

.parent{
            width: 200px;
            height: 200px;
            background-color: red;
            position: relative;
        }
        .son{
            width: 20px;
            height: 20px;
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
            margin: auto;
        }