string数组排序

想要对一个string数组排序
原来的数组是这样{"bb","bb","fff","a","ffff","c","dd","f","dd","e","dd"}
现在对这个数组进行排序，排序的规则是只出现一次的排在前面，出现2次及2次
以上的排在后面，那么就上面的数组排序后想要的结果就是
{"a","c","e","f","bb","bb","dd","dd","dd","fff","ffff"}

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