根据字符的频率对字符串进行排序

给定字符串str ，任务是根据每个字符的频率以升序对字符串进行排序。如果两个元素具有相同的频率，则将它们按字典顺序排序。
例子：

Input: str = “geeksforgeeks” 
Output: forggkksseeee 
Explanation: 
Frequency of characters: g2 e4 k2 s2 f1 o1 r1 
Sorted characters according to frequency: f1 o1 r1 g2 k2 s2 e4 
f, o, r occurs one time so they are ordered lexicographically and so are g, k and s. 
Hence the final output is forggkksseeee.
Input: str = “abc” 
Output: abc 

方法的想法是将每个字符及其频率存储在成对的向量中，然后根据存储的频率对向量对进行排序。最后，按顺序打印矢量。
下面是上述方法的实现：

C++

// C++ implementation to Sort strings
// according to the frequency of
// characters in ascending order
 
#include 
using namespace std;
 
// Returns count of character in the string
int countFrequency(string str, char ch)
{
    int count = 0;
 
    for (int i = 0; i < str.length(); i++)
 
        // Check for vowel
        if (str[i] == ch)
            ++count;
 
    return count;
}
 
// Function to sort the string
// according to the frequency
void sortArr(string str)
{
    int n = str.length();
 
    // Vector to store the frequency of
    // characters with respective character
    vector > vp;
 
    // Inserting frequency
    // with respective character
    // in the vector pair
    for (int i = 0; i < n; i++) {
 
        vp.push_back(
            make_pair(
                countFrequency(str, str[i]),
                str[i]));
    }
 
    // Sort the vector, this will sort the pair
    // according to the number of characters
    sort(vp.begin(), vp.end());
 
    // Print the sorted vector content
    for (int i = 0; i < vp.size(); i++)
        cout << vp[i].second;
}
 
// Driver code
int main()
{
    string str = "geeksforgeeks";
 
    sortArr(str);
 
    return 0;
}

Python3

# Python3 implementation to Sort strings
# according to the frequency of
# characters in ascending order
 
# Returns count of character in the string
def countFrequency(string ,  ch) :
 
    count = 0;
 
    for i in range(len(string)) :
 
        # Check for vowel
        if (string[i] == ch) :
            count += 1;
 
    return count;
 
# Function to sort the string
# according to the frequency
def sortArr(string) :
    n = len(string);
 
    # Vector to store the frequency of
    # characters with respective character
    vp = [];
 
    # Inserting frequency
    # with respective character
    # in the vector pair
    for i in range(n) :
 
        vp.append((countFrequency(string, string[i]), string[i]));
         
    # Sort the vector, this will sort the pair
    # according to the number of characters
    vp.sort();
     
    # Print the sorted vector content
    for i in range(len(vp)) :
        print(vp[i][1],end="");
 
# Driver code
if __name__ == "__main__" :
 
    string = "geeksforgeeks";
 
    sortArr(string);
 
    # This code is contributed by Yash_R

输出：

forggkksseeee

时间复杂度： O(n * log(n))

辅助空间： O(n)