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All leaked interview problems are collected from Internet.

All leaked interview problems are collected from Internet.

Given a string, sort it in decreasing order based on the frequency of characters.

**Example 1:**

Input:"tree"Output:"eert"Explanation:'e' appears twice while 'r' and 't' both appear once. So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

**Example 2:**

Input:"cccaaa"Output:"cccaaa"Explanation:Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer. Note that "cacaca" is incorrect, as the same characters must be together.

**Example 3:**

Input:"Aabb"Output:"bbAa"Explanation:"bbaA" is also a valid answer, but "Aabb" is incorrect. Note that 'A' and 'a' are treated as two different characters.

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