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.

这道题让我们给一个字符串按照字符出现的频率来排序，那么毫无疑问肯定要先统计出每个字符出现的个数，那么之后怎么做呢？我们可以利用优先队列的自动排序的特点，把个数和字符组成pair放到优先队列里排好序后，再取出来组成结果res即可，参见代码如下：

解法一：

class Solution {
public:
    string frequencySort(string s) {
        string res = "";
        priority_queue<pair<int, char>> q;
        unordered_map<char, int> m;
        for (char c : s) ++m[c];
        for (auto a : m) q.push({a.second, a.first});
        while (!q.empty()) {
            auto t = q.top(); q.pop();
            res.append(t.first, t.second);
        }
        return res;
    }
};

我们也可以使用STL自带的sort来做，关键就在于重写comparator，由于需要使用外部变量，记得中括号中放入＆，然后我们将频率大的返回，注意一定还要处理频率相等的情况，要不然两个频率相等的字符可能穿插着出现在结果res中，这样是不对的。参见代码如下：

解法二：

class Solution {
public:
    string frequencySort(string s) {
        unordered_map<char, int> m;
        for (char c : s) ++m[c];
        sort(s.begin(), s.end(), [&](char& a, char& b){
            return m[a] > m[b] || (m[a] == m[b] && a < b);
        });
        return s;
    }
};

我们也可以不使用优先队列，而是建立一个字符串数组，因为某个字符的出现次数不可能超过s的长度，所以我们将每个字符根据其出现次数放入数组中的对应位置，那么最后我们只要从后往前遍历数组所有位置，将不为空的位置的字符串加入结果res中即可，参见代码如下：

解法三：

class Solution {
public:
    string frequencySort(string s) {
        string res = "";
        vector<string> v(s.size() + 1, "");
        unordered_map<char, int> m;
        for (char c : s) ++m[c];
        for (auto& a : m) {
            v[a.second].append(a.second, a.first);
        }
        for (int i = s.size(); i > 0; --i) {
            if (!v[i].empty()) {
                res.append(v[i]);
            }
        }
        return res;
    }
};

类似题目：

Top K Frequent Words

First Unique Character in a String

参考资料：

https://leetcode.com/problems/sort-characters-by-frequency/description/

https://leetcode.com/problems/sort-characters-by-frequency/discuss/93404/c-on-solution-without-sort

https://leetcode.com/problems/sort-characters-by-frequency/discuss/93409/concise-c-solution-using-stl-sort