Loop

for

Basic for loop

for (int i = 0; i < 5; i++)
{
    Console.WriteLine(i);  // 0, 1, 2, 3, 4
}

Reverse iteration

for (int i = 4; i >= 0; i--)
{
    Console.WriteLine(i);  // 4, 3, 2, 1, 0
}

Step increment

for (int i = 0; i < 10; i += 2)
{
    Console.WriteLine(i);  // 0, 2, 4, 6, 8
}

foreach

Iterate array

int[] numbers = { 1, 2, 3, 4, 5 };
foreach (int num in numbers)
{
    Console.WriteLine(num);
}

Iterate List

List<string> names = new List<string> { "Alice", "Bob" };
foreach (string name in names)
{
    Console.WriteLine(name);
}

Iterate Dictionary

Dictionary<string, int> dict = new Dictionary<string, int>
{
    { "one", 1 },
    { "two", 2 }
};
foreach (KeyValuePair<string, int> kvp in dict)
{
    Console.WriteLine($"{kvp.Key}: {kvp.Value}");
}

while / do-while

while - 先判斷再執行

int i = 0;
while (i < 5)
{
    Console.WriteLine(i);  // 0, 1, 2, 3, 4
    i++;
}

do-while - 先執行再判斷，至少執行一次

int j = 0;
do
{
    Console.WriteLine(j);  // 0, 1, 2, 3, 4
    j++;
} while (j < 5);

break / continue

break - 跳出迴圈

for (int i = 0; i < 10; i++)
{
    if (i == 5) break;
    Console.WriteLine(i);  // 0, 1, 2, 3, 4
}

continue - 跳過本次，繼續下一次

for (int i = 0; i < 5; i++)
{
    if (i == 2) continue;
    Console.WriteLine(i);  // 0, 1, 3, 4
}

Recursive

Factorial 階乘遞迴: 一般遞迴，堆疊累積

int Factorial(int n)
{
    if (n <= 1) return 1;
    return n * Factorial(n - 1);
}
// Factorial(5) = 5 * 4 * 3 * 2 * 1 = 120

Fibonacci 斐波那契數列遞迴: 值等於前兩項相加

int Fibonacci(int n)
{
    if (n <= 1) return n;
    return Fibonacci(n - 1) + Fibonacci(n - 2);
}
// Fibonacci(6) = 8  (0, 1, 1, 2, 3, 5, 8)

FactorialTail 尾遞迴: 累加器攜帶中間結果，不累積堆疊

int FactorialTail(int n, int acc = 1)
{
    if (n <= 1) return acc;
    return FactorialTail(n - 1, n * acc);
}