图片适应屏幕居中显示，且不变形

html：

<div class=‘item‘>
    <div class = ‘container‘ />
</div>

css:

    .item {
        width: 100%;
        height: 100%;
           .container  {
              max-width: 100%;
              height: auto;
              min-height: 720px;
              position: absolute;
              left: 50%;
              top: 50%;
              transform: translate(-50%, -50%);  // 利用位移实现居中                               
              background-image: url(./img/1.png);
              background-size: 100%;
              background-repeat: no-repeat;
              background-position: center 0;
    }