Flexbox-如果有1个项目，则居中对齐；如果有2个项目，则将居中对齐

margin-top: auto 在这里是正确的事情：

.flex {
  display: flex;
  flex-direction: column;
  justify-content: center;

  /* for demonstration purposes */
  border: 1px solid black;
  min-height: 200px;
  margin-bottom: 10px;
}

.flex > div:not(:only-child):last-of-type {
  margin-top: auto;
}

  
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