LeetCode Link: 451. Sort Characters By Frequency
Language: C#
Problem Statement
Given a string s, sort it in decreasing order based on the frequency of the characters. The frequency of a character is the number of times it appears in the string.
Return the sorted string. If there are multiple answers, return any of them.
Examples
Example 1:
Input: s = “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: s = “cccaaa” Output: “aaaccc” Explanation: Both ‘c’ and ‘a’ appear three times, so both “cccaaa” and “aaaccc” are valid answers. Note that “cacaca” is incorrect, as the same characters must be together. Example 3:
Input: s = “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.
Constraints
- 1 <= s.length <= 5 * 105
- s consists of uppercase and lowercase English letters and digits.
Solution
public class Solution
{
public string FrequencySort(string s)
{
var pq = new PriorityQueue<char, int>();
var dict = new Dictionary<char, int>();
foreach (var c in s)
{
if (!dict.ContainsKey(c))
{
dict[c] = 0;
}
dict[c] = dict[c] - 1;
}
foreach (var k in dict.Keys)
{
pq.Enqueue(k, dict[k]);
}
var sb = new StringBuilder();
while (pq.Count > 0)
{
var c = pq.Dequeue();
var k = Math.Abs(dict[c]);
for (int i=0; i<k; i++)
{
sb.Append(c);
}
}
return sb.ToString();
}
}
Complexity
Time Complexity: O(N)
Space Complexity: O(N)