Fix: prevent out-of-bounds write in counting sort for strings

sorting/counting_sort_string.cpp

@@ -1,33 +1,66 @@
 // C++ Program for counting sort
 #include <iostream>
-
+#include <string>
+#include <vector>
 using namespace std;
 
-void countSort(string arr) {
-    string output;
+void countSort(string& arr) {
+    int n = static_cast<int>(arr.length());
+
+    // Handle edge cases first
+    if (n == 0) {
+        cout << "Empty String\n";
+        return;
+    }
+
+    if (n == 1) {
+        cout << "Already Sorted " << arr << '\n';
+        return;
+    }
 
-    int count[256], i;
-    for (int i = 0; i < 256; i++) count[i] = 0;
+    // output string preallocated with correct size
+    string output(n, '\0');
 
-    for (i = 0; arr[i]; ++i) ++count[arr[i]];
+    // use 256 for all possible unsigned char values
+    vector<int> count(256, 0);
 
-    for (i = 1; i < 256; ++i) count[i] += count[i - 1];
+    // store count of each character
+    for (int i = 0; i < n; ++i) {
+        ++count[static_cast<unsigned char>(arr[i])];
+    }
 
-    for (i = 0; arr[i]; ++i) {
-        output[count[arr[i]] - 1] = arr[i];
-        --count[arr[i]];
+    // change count[i] so that count[i] now contains actual
+    // position of this character in output
+    for (int i = 1; i < 256; ++i) {
+        count[i] += count[i - 1];
     }
 
-    for (i = 0; arr[i]; ++i) arr[i] = output[i];
+    // build the output array (iterate from end for stable sort)
+    for (int i = n - 1; i >= 0; --i) {
+        unsigned char ch = static_cast<unsigned char>(arr[i]);
+        int pos = count[ch] - 1;  // position index in output
+        output[pos] = arr[i];
+        --count[ch];
+    }
 
-    cout << "Sorted character array is " << arr;
+    // copy output to arr
+    arr = output;
+    cout << "Sorted character array is " << arr << '\n';
 }
 
 int main() {
-    string arr;
-    cin >> arr;
+    string s1 = "hello";
+    countSort(s1);  // Sorted character array is ehllo
+
+    string s2 = "AaBbZz012";
+    countSort(s2); // Sorted character array is 012ABZabz
 
-    countSort(arr);
+    string s3 = "";
+    countSort(s3); // Empty String
+
+    string s4 = "x";
+    countSort(s4); // Already Sorted x
 
     return 0;
 }
+