Added Q451

Author: nishant0708

dsa-solutions/lc-solutions/0400-0499/0451-Sort-Characters-By-Frequency.md

@@ -0,0 +1,149 @@
+---
+id: sort-characters-by-frequency
+title: Sort Characters By Frequency
+sidebar_label: 0451-Sort-Characters-By-Frequency
+tags:
+- Hash Table
+- String
+- Sorting
+- Heap (Priority Queue)
+description: "Given a string `s`, sort it in decreasing order based on the frequency of characters, and return the sorted string."
+---
+
+## Problem
+
+Given a string `s`, sort it in decreasing order based on the frequency of characters, and return the sorted string.
+
+### Examples
+
+**Example 1:**
+
+**Input:** `s = "tree"`  
+**Output:** `"eert"`  
+**Explanation:** 'e' appears twice while 'r' and 't' both appear once. So 'e' must appear before 'r' and 't'. Therefore "eetr" is also a valid answer.
+
+**Example 2:**
+
+**Input:** `s = "cccaaa"`  
+**Output:** `"cccaaa"`  
+**Explanation:** Both 'c' and 'a' appear three times. So "aaaccc" is also a valid answer. Note that "cacaca" is incorrect, as same characters must be together.
+
+**Example 3:**
+
+**Input:** `s = "Aabb"`  
+**Output:** `"bbAa"`  
+**Explanation:** 'b' appears twice, 'A' and 'a' both appear once. Note that 'A' and 'a' are treated as two different characters.
+
+### Constraints
+
+- `1 <= s.length <= 5 * 10^5`
+- `s` consists of uppercase and lowercase English letters and digits.
+
+---
+
+## Approach
+
+To solve this problem, we can follow these steps:
+
+1. **Count Frequencies:** Use a hash table (dictionary) to count the frequency of each character in the string.
+2. **Sort Characters:** Create a list of characters sorted by their frequency in descending order.
+3. **Build Result String:** Construct the result string by repeating characters based on their frequencies.
+
+### Steps:
+
+1. Create a hash table to store the frequency of each character.
+2. Sort the characters based on their frequency in descending order.
+3. Build the final string by appending each character multiplied by its frequency.
+
+### Solution
+
+#### Java Solution
+
+```java
+import java.util.HashMap;
+import java.util.Map;
+import java.util.PriorityQueue;
+
+class Solution {
+    public String frequencySort(String s) {
+        Map<Character, Integer> frequencyMap = new HashMap<>();
+        for (char c : s.toCharArray()) {
+            frequencyMap.put(c, frequencyMap.getOrDefault(c, 0) + 1);
+        }
+        
+        PriorityQueue<Character> maxHeap = new PriorityQueue<>((a, b) -> frequencyMap.get(b) - frequencyMap.get(a));
+        maxHeap.addAll(frequencyMap.keySet());
+        
+        StringBuilder result = new StringBuilder();
+        while (!maxHeap.isEmpty()) {
+            char c = maxHeap.poll();
+            int count = frequencyMap.get(c);
+            for (int i = 0; i < count; i++) {
+                result.append(c);
+            }
+        }
+        
+        return result.toString();
+    }
+}
+```
+### C++ Solution
+
+```cpp 
+#include <unordered_map>
+#include <queue>
+#include <string>
+
+class Solution {
+public:
+    string frequencySort(string s) {
+        unordered_map<char, int> frequencyMap;
+        for (char c : s) {
+            frequencyMap[c]++;
+        }
+        
+        priority_queue<pair<int, char>> maxHeap;
+        for (auto& [c, freq] : frequencyMap) {
+            maxHeap.push({freq, c});
+        }
+        
+        string result;
+        while (!maxHeap.empty()) {
+            auto [freq, c] = maxHeap.top();
+            maxHeap.pop();
+            result.append(freq, c);
+        }
+        
+        return result;
+    }
+};
+```
+
+### Python Solution
+
+```python
+from collections import Counter
+import heapq
+
+class Solution:
+    def frequencySort(self, s: str) -> str:
+        frequency_map = Counter(s)
+        max_heap = [(-freq, char) for char, freq in frequency_map.items()]
+        heapq.heapify(max_heap)
+        
+        result = []
+        while max_heap:
+            freq, char = heapq.heappop(max_heap)
+            result.append(char * -freq)
+        
+        return ''.join(result)
+```
+### Complexity Analysis
+**Time Complexity:** O(n log n)
+>Reason: Counting the frequency of each character takes O(n), and sorting the characters based on frequency takes O(n log n).
+
+**Space Complexity:** O(n)
+>Reason: The space needed for the frequency map, heap, and result string is proportional to the length of the input string.
+
+### References
+LeetCode Problem: Sort Characters By Frequency