almost lost everything there :/

3/index.md

@@ -7,13 +7,11 @@ Suppose we would like to implement a queue system for a commercial website.
 Given that the website offers a list of items, and each item has a limited quantity in stock, implement a queue of customers, such that each customer can buy only a few products among the available items.
 
 - Determine the item name, quantity, and price in stock.
-- Specify a queue of n customers; each customer can buy k products from one
+- Specify a queue of n customers; each customer can buy k products from one item (k is different for each customer).
-item (k is different for each customer).
 - Customers take turns to enter (enqueue) and leave (dequeue) the queue according to the FIFO order to purchase wanted products.
 - If a customer successfully purchases products, display a message and reduce the number of products in stock. Otherwise, display a warning message if the item has been run out.
 
-Implement the above problems in C/C++ using a Queue data structure. Write
+Implement the above problems in C/C++ using a Queue data structure. Write a main function for testing all written functions (init(), display(), enqueue(), dequeue(),...).
-a main function for testing all written functions (init(), display(), enqueue(), dequeue(),...).
 
 ## ex 2
 
@@ -24,7 +22,6 @@ Choose one method to implement a Stack data structure (Array-based or Stack usin
 - A website may contain a url and a title; suppose that we have visited a list of websites.
 - The order of visited websites is stored in a Backward stack according to FILO.
 - If we return to the previous website, we pop the Backward stack and push it into the Forward stack.
-- And otherwise, when we forward to the next website, we retrieve the top
+- And otherwise, when we forward to the next website, we retrieve the top website from the Forward stack and push it into the Backward stack.
-website from the Forward stack and push it into the Backward stack.
 
 Write a main function to test all written functions (init(), display(), push(), pop(),...).

4/digitSum.c

@@ -0,0 +1,36 @@
+#include <stdio.h>
+
+int calculate_iter(int n)
+{
+    int c = 0;
+    while (n > 0)
+    {
+        c += n % 10;
+        n /= 10;
+    }
+    return c;
+}
+
+int calculate_recur(int n)
+{
+    if (n <= 0)
+        return 0;
+    return n % 10 + calculate_iter(n / 10);
+}
+
+// both complexity O(logn)
+
+int main(int argc, char const *argv[])
+{
+    int w;
+    printf("number: ");
+    scanf("%d", &w);
+    int a = calculate_iter(w);
+    int b = calculate_recur(w);
+    if (a != b)
+        printf("oops! %d != %d\n", a, b);
+    else
+        printf(" yay! %d == %d\n", a, b);
+    // main(argc, argv);
+    return 0;
+}

4/sphenic_recur.c

@@ -0,0 +1,53 @@
+#include <stdio.h>
+#include <stdbool.h>
+
+bool _sphenic(int num, int cnt, int pr)
+{
+    // Base case: If we have already found 3 distinct primes, check if the number is fully divided.
+    if (cnt == 3)
+        return num == 1;
+
+    // Try dividing the number by the next prime factor.
+    while (++pr <= num)
+    {
+        if (num % pr == 0)
+        {
+            num /= pr;
+
+            // If the same prime factor divides it again, it's not sphenic.
+            if (num % pr == 0)
+                return false;
+
+            // Recursively check for the next prime and increase the count of distinct primes.
+            return _sphenic(num, cnt + 1, pr);
+        }
+    }
+
+    // If we run out of primes without finding 3 distinct ones, it's not sphenic.
+    return false;
+}
+
+bool rsphenic(int n)
+{
+    // Sphenic numbers are always greater than or equal to 30.
+    if (n < 30)
+        return false;
+
+    // Initialize the recursion to start with prime checking from 2.
+    return _sphenic(n, 0, 1);
+}
+
+int main()
+{
+    int N = 200;
+
+    for (int i = 1; i < N; i++)
+    {
+        // printf("%d\n", i);
+
+        if (rsphenic(i))
+            printf("%d ", i);
+    }
+
+    return 0;
+}

specials/1.md

@@ -0,0 +1,20 @@
+# obj
+
+verify whether a set of 3 values is a Pythagorean triple. A Pythagorean triple is a set of 3 positive numbers, which are expressed as a2 + b2 = c2.
+
+Example:
+
+- (3, 5, 4) is a Pythagorean triple, 3^2 + 4^2 = 5^2.
+- (10, 8, 6) is a Pythagorean triple, 6^2 + 8^2 = 10^2.
+- (6, 8, 13) is not a Pythagorean triple.
+
+## 1
+
+- Write a pseudo-code to find all Pythagorean triples from a given array using Iteration.
+- Implement your proposed algorithm in C/C++. You have to initialize a random array (any size >= 6, any value) in the main() function.
+- Calculate the complexity of your algorithm. Justify the answer (comment directly in your source files).
+
+## 2
+
+- Propose another algorithm using recursive functions and implement in C/C++ to perform the above problem.
+- Calculate the complexity of your algorithm. Justify the answer (comment directly in your source files).

specials/2.md

@@ -0,0 +1,50 @@
+# i especially hate this one
+
+## prompt
+
+In this problem, we try to convert a decimal fraction into binary number. In the input, we are given an fraction decimal number n and integer k, convert decimal number n into equivalent binary number up-to k precision after decimal point.
+
+We have to follow the algorithm:
+
+1. Convert the integral part of decimal to binary equivalent:
+
+   - Divide the decimal number by 2 and store remainders in array.
+   - Divide the quotient by 2.
+   - Repeat step 2 until we get the quotient equal to zero.
+   - Equivalent binary number would be reverse of all remainders of step 1.
+
+2. Convert the fractional part of decimal to binary equivalent
+
+   - Multiply the fractional decimal number by 2.
+   - Integral part of resultant decimal number will be first digit of fraction binary number.
+   - Repeat step 1 using only fractional part of decimal number and then step 2.
+
+Let's take an example for n = 4.47, k = 4
+Step 1: Conversion of 4 to binary
+
+1. 4/2 : Remainder = 0 : Quotient = 2
+2. 2/2 : Remainder = 0 : Quotient = 1
+3. 1/2 : Remainder = 1 : Quotient = 0
+
+So equivalent binary of integral part of decimal is 100.
+
+Step 2: Conversion of .47 to binary
+
+1. 0.47 \* 2 = 0.94, Integral part: 0
+2. 0.94 \* 2 = 1.88, Integral part: 1
+3. 0.88 \* 2 = 1.76, Integral part: 1
+4. 0.76 \* 2 = 1.52, Integral part: 1
+
+So equivalent binary of fractional part of decimal is .0111  
+The final result of conversion n = 4.47 (with precision k = 4): 100.0111
+
+## 1
+
+- Write a pseudo-code to convert a natural number into binary using Iteration.
+- Implement your proposed algorithm in C/C++.
+- Calculate the complexity of your algorithm. Justify the answer (comment directly in your source files).
+
+## 2
+
+- Propose another algorithm using recursive functions and implement in C/C++ to perform the above problem.
+- Calculate the complexity of your algorithm. Justify the answer (comment directly in your source files).

specials/3.md

@@ -0,0 +1,55 @@
+# id rather use chatgpt
+
+## 1
+
+Initialize a random array of your choice. The current goal is to verify whether an array number is
+centered cube.
+Note: a centered cube number is calculated by the following formula:
+𝑓(𝑛) = 𝑛
+3 + (𝑛 + 1)
+3
+For example, the centered cube numbers are:
+f(1) = 1
+3 + (1 + 1)
+3 = 9
+f(2) = 2
+3 + (2 + 1)
+3 = 8 + 27 = 35
+f(3) = 3
+3 + (3 + 1)
+3 = 27 + 64 = 91
+...
+
+- Write pseudo-code using Recursion (if necessary, Iteration) to check all values in the given
+  array are centered cube or not.
+- Write a program in C/C++ to complete your proposed algorithm in the pseudo-code. .
+  Note: The program must include a recursive function and the main function.
+- Calculate the complexity of your algorithm. Justify the answer.
+
+## 2
+
+This problem requires to traverse and display a tree using the following process:
+
+- Visit a node and find its child nodes.
+- Put the child nodes into the queue.
+- Pop a node in the queue and repeat the process until all nodes are visited.
+  Justify and print the traversal result for the below tree.
+  10
+  4 8
+  13 20 90
+
+## 3
+
+Given an array of n elements, the Bubble sort algorithm swaps repeatedly the adjacent elements
+if they are in the wrong order (ascending order). We want to implement an improve version of this
+sorting algorithm.
+The idea is to sort both maximum (at the end) and minimum (at the beginning) in one pass and the
+array is sorted from both ends.
+
+- Traverse from left to right and compare adjacent elements and the maximum is placed at
+  right side.
+- Traverse from right to left and compare adjacent elements and the minimum is placed at
+  left side.
+- Repeat the process until the array is sorted.
+  Implement this algorithm in C/C++. Calculate the complexity of the above algorithm. Justify your
+  answer.