Question Definition
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.
Java Solution
public String frequencySort(String s) {
Map<Character, Integer> map = new HashMap<>();
for(char c : s.toCharArray()){
map.putIfAbsent(c, 0);
map.put(c, map.get(c) + 1);
}
map = map.entrySet().stream().sorted((e1, e2) ->
e2.getValue() - e1.getValue())
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
(e1, e2) -> e1, LinkedHashMap::new));
StringBuilder sb = new StringBuilder();
for(Map.Entry<Character, Integer> entry : map.entrySet()){
int times = entry.getValue();
while(times-- > 0){
sb.append(entry.getKey());
}
}
return sb.toString();
}
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