tes("abcd")); // Output: "abcd" } } 28:T4d3,#include #include #include class Solution { public: std::string removeDuplicates(const std::string& s) { // Initialize stack std::stack stack; // Process each character in s for (char c : s) { // If the stack is not empty and the current character is the same as the top of the stack, pop the character from the stack if (!stack.empty() && c == stack.top()) { stack.pop(); } else { // Push the current character onto the stack stack.push(c); } } // Convert stack to string std::string result; while (!stack.empty()) { result.push_back(stack.top()); stack.pop(); } // Reverse the string to maintain the order std::reverse(result.begin(), result.end()); return result; } }; int main() { Solution solution; std::cout << solution.removeDuplicates("abccba") << std::endl; // Output: "" std::cout << solution.removeDuplicates("foobar") << std::endl; // Output: "fbar" std::cout << solution.removeDuplicates("abcd") << std::endl; // Output: "abcd" return 0; } 29:T45b,using System; using System.Collections.Generic; public class Solution { public string RemoveDuplicates(string s) { // Initialize stack Stack stack = new Stack(); // Process each character in s foreach (char c in s) { // If the stack is not empty and the current character is the same as the top of the stack, pop the character from the stack if (stack.Count > 0 && c == stack.Peek()) { stack.Pop(); } else { // Push the current character onto the stack stack.Push(c); } } // Convert stack to string char[] resultArray = stack.ToArray(); Array.Reverse(resultArray); retur