SOURCE

<div class="wrap">
  1 父元素
  <div class="left">
    2 左栏
  </div>
  <div class="right">
    3 右栏
  </div>
</div>
<div class="footer">
  4 底栏
</div>

body {float:left;}
div {
  padding: 10px;
}.wrap {
  border: 2px solid purple;
  float:left;
}.left {
  width: 50px;
  float: left;
  border: 2px solid green;

}

.right {
  width: 50px;
  float: right;
  border: 2px solid yellow;
}.footer {
  border: 2px solid cyan;
}