css高度已知，左右定宽，中间自适应三栏布局：
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta http-equiv="X-UA-Compatible" content="ie=edge"><title>Document</title><style>*{margin: 0;padding: 0;}.box>div{height: 100px;}.box .left{background-color: red;width: 300px;float: left;}.box .right{background-color: blue;width: 300px;float: right;}.box .center{text-align: center;line-height: 100px;color: #fff;background-color: orange;}</style>
</head>
<body><div class="box"><div class="left"></div><div class="right"></div><div class="center">第一种方法：浮动</div></div></body>
</html>
![bzoj 5248: [2018多省省队联测]一双木棋](http://pic.xiahunao.cn/bzoj 5248: [2018多省省队联测]一双木棋)
