如何在div的底部和中心放置按钮？

<div class="parent">
  <button class="btn">按钮</button>
</div>

.parent {
  display: flex;
  justify-content: center;
  flex-direction: column;
  height: 100vh; /* 设置父div的高度以占满整个视口 */
}

.btn {
  margin-top: auto;
}

<div class="parent">
  <button class="btn">按钮</button>
</div>

.parent {
  position: relative;
  height: 100vh; /* 设置父div的高度以占满整个视口 */
}

.btn {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, 50%);
}