451. Sort Characters By Frequency

Problem:

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.

思路：

Solution (C++):

string frequencySort(string s) {
    if (s.empty())  return "";
    int n = s.length();
    vector<int> count(256, 0);
    
    for (char c : s)  ++count[c];
    
    sort(s.begin(), s.end(), [&](char a, char b) {
        return (count[a] > count[b]) || (count[a] == count[b] && a < b); });
    return s;
}

性能：

Runtime: 104 ms  Memory Usage: 8.2 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB