diff --git a/.DS_Store b/.DS_Store
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diff --git a/Answers/HowToSend.txt b/40130212009/HowToSend.txt
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rename to 40130212009/HowToSend.txt
diff --git a/stack.cpp b/stack.cpp
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+++ b/stack.cpp
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+#include <iostream>
+#include <vector>
+#include <stack>
+#include <string>
+#include <cmath>
+#include <unordered_map>
+#include <algorithm>
+#include <sstream>
+
+class Stack {
+private:
+    std::vector<int> items;
+
+public:
+    void push(int item) {
+        items.push_back(item);
+    }
+
+    bool isEmpty() const {
+        return items.empty();
+    }
+
+    int pop() {
+        if (isEmpty()) {
+            return -1; // Return -1 to indicate an empty stack
+        }
+        int item = items.back();
+        items.pop_back();
+        return item;
+    }
+
+    int peek() const {
+        if (isEmpty()) {
+            return -1; // Return -1 to indicate an empty stack
+        }
+        return items.back();
+    }
+
+    int size() const {
+        return items.size();
+    }
+
+    int getMax() const {
+        if (isEmpty()) {
+            return -1; // Return -1 to indicate an empty stack
+        }
+        return *std::max_element(items.begin(), items.end());
+    }
+};
+
+std::string reverseString(const std::string &inputString) {
+    Stack s;
+    for (char ch : inputString) {
+        s.push(ch);
+    }
+
+    std::string reversedString;
+    while (!s.isEmpty()) {
+        reversedString += s.pop();
+    }
+
+    return reversedString;
+}
+
+bool isOperator(char ch) {
+    return ch == '*' || ch == '-' || ch == '+' || ch == '/' || ch == '^';
+}
+
+int evaluatePostfix(const std::string &expression) {
+    Stack s;
+    std::stringstream ss(expression);
+    std::string token;
+    while (ss >> token) {
+        if (isOperator(token[0]) && token.length() == 1) {
+            int operand2 = s.pop();
+            int operand1 = s.pop();
+            int result = 0;
+            switch (token[0]) {
+                case '*':
+                    result = operand1 * operand2;
+                    break;
+                case '-':
+                    result = operand1 - operand2;
+                    break;
+                case '+':
+                    result = operand1 + operand2;
+                    break;
+                case '/':
+                    if (operand2 == 0) {
+                        throw std::runtime_error("Division by zero");
+                    }
+                    result = operand1 / operand2;
+                    break;
+                case '^':
+                    result = std::pow(operand1, operand2);
+                    break;
+            }
+            s.push(result);
+        } else {
+            s.push(std::stoi(token));
+        }
+    }
+
+    return s.pop();
+}
+
+bool isBalanced(const std::string &expression) {
+    std::unordered_map<char, char> matching = {{')', '('}, {'}', '{'}, {']', '['}};
+    Stack s;
+    for (char ch : expression) {
+        if (matching.count(ch) == 0) {
+            s.push(ch);
+        } else {
+            if (s.isEmpty() || s.pop() != matching[ch]) {
+                return false;
+            }
+        }
+    }
+    return s.isEmpty();
+}
+
+std::string prefixToPostfix(const std::string &expression) {
+    Stack s;
+    std::stringstream ss(expression);
+    std::vector<std::string> tokens;
+    std::string token;
+
+    while (ss >> token) {
+        tokens.push_back(token);
+    }
+
+    std::reverse(tokens.begin(), tokens.end());
+
+    for (const std::string &token : tokens) {
+        if (isOperator(token[0]) && token.length() == 1) {
+            std::string operand1 = s.pop();
+            std::string operand2 = s.pop();
+            std::string postExp = operand1 + " " + operand2 + " " + token;
+            s.push(postExp);
+        } else {
+            s.push(token);
+        }
+    }
+
+    return s.pop();
+}
+
+Stack sortStack(Stack stack) {
+    Stack tempStack;
+    while (!stack.isEmpty()) {
+        int temp = stack.pop();
+        while (!tempStack.isEmpty() && temp < tempStack.peek()) {
+            stack.push(tempStack.pop());
+        }
+        tempStack.push(temp);
+    }
+    while (!tempStack.isEmpty()) {
+        stack.push(tempStack.pop());
+    }
+    return stack;
+}
+
+int precedence(char op) {
+    if (op == '+' || op == '-') {
+        return 1;
+    }
+    if (op == '*' || op == '/') {
+        return 2;
+    }
+    return 0;
+}
+
+std::string infixToPostfix(const std::string &expression) {
+    std::vector<std::string> output;
+    Stack stack;
+    std::stringstream ss(expression);
+    std::string token;
+
+    while (ss >> token) {
+        if (token == "(") {
+            stack.push(token[0]);
+        } else if (token == ")") {
+            while (!stack.isEmpty() && stack.peek() != '(') {
+                output.push_back(std::string(1, stack.pop()));
+            }
+            stack.pop();
+        } else if (isOperator(token[0]) && token.length() == 1) {
+            while (!stack.isEmpty() && precedence(stack.peek()) >= precedence(token[0])) {
+                output.push_back(std::string(1, stack.pop()));
+            }
+            stack.push(token[0]);
+        } else {
+            output.push_back(token);
+        }
+    }
+
+    while (!stack.isEmpty()) {
+        output.push_back(std::string(1, stack.pop()));
+    }
+
+    std::string result;
+    for (const std::string &str : output) {
+        if (!result.empty()) {
+            result += " ";
+        }
+        result += str;
+    }
+
+    return result;
+}
+
+std::vector<int> dailyTemperatures(const std::vector<int> &temperatures) {
+    int n = temperatures.size();
+    std::vector<int> result(n, 0);
+    Stack stack;
+
+    for (int i = 0; i < n; ++i) {
+        while (!stack.isEmpty() && temperatures[stack.peek()] < temperatures[i]) {
+            int index = stack.pop();
+            result[index] = i - index;
+        }
+        stack.push(i);
+    }
+
+    return result;
+}
+
+int longestValidParentheses(const std::string &str) {
+    Stack stack;
+    stack.push(-1);
+    int maxLength = 0;
+
+    for (int i = 0; i < str.length(); ++i) {
+        char ch = str[i];
+        if (ch == '(') {
+            stack.push(i);
+        } else {
+            stack.pop();
+            if (stack.isEmpty()) {
+                stack.push(i);
+            } else {
+                maxLength = std::max(maxLength, i - stack.peek());
+            }
+        }
+    }
+
+    return maxLength;
+}
+
+int main() {
+    std::string inputString = "hello";
+    std::cout << "Reversed string: " << reverseString(inputString) << std::endl;
+
+    std::string postfixExpr = "2 3 * 5 4 * + 9 -";
+    std::cout << "Postfix evaluation: " << evaluatePostfix(postfixExpr) << std::endl;
+
+    std::string balancedExpr = "{[()]}";
+    std::cout << "Is balanced: " << (isBalanced(balancedExpr) ? "Yes" : "No") << std::endl;
+
+    std::string prefixExpr = "* + 2 3 4";
+    std::cout << "Prefix to Postfix: " << prefixToPostfix(prefixExpr) << std::endl;
+
+    Stack stack;
+    stack.push(3);
+    stack.push(1);
+    stack.push(4);
+    stack.push(2);
+    stack = sortStack(stack);
+    std::cout << "Sorted stack: ";
+    while (!stack.isEmpty()) {
+        std::cout << stack.pop() << " ";
+    }
+    std::cout << std::endl;
+
+    std::string infixExpr = "3 + 5 * 2 - 8 / 4";
+    std::cout << "Infix to Postfix: " << infixToPostfix(infixExpr) << std::endl;
+
+    std::vector<int> temperatures = {73, 74, 75, 71, 69, 72, 76, 73};
+    std::vector<int> tempResults = dailyTemperatures(temperatures);
+    std::cout << "Daily temperatures: ";
+    for (int days : tempResults) {
+        std::cout << days << " ";
+    }
+    std::cout << std::endl;
+
+    std::string parenStr = "(()())";
+    std::cout << "Longest valid parentheses: " << longestValidParentheses(parenStr) << std::endl;
+
+    return 0;
+}