函数递归

函数main()在下面例子里调用函数up_and_down()，我们称这为“第一层递归”，然后up_and_down()调用自己，我们称之为“第二层递归”，第二层调用第三层，以此类推。

#include <stdio.h>
void up_and_down(int);
int main(void) {
  up_and_down(1);
  return 0;
}
void up_and_down(int n) {
  printf("Level %d: n location %p\n", n, &n);
  if (n < 4)
    up_and_down(n + 1);
  // this line will be executed certainly in the end in this method
  printf("LEVEL %d: n location %p\n", n, &n);
}

Level 1: n location 0x0012ff48
Level 2: n location 0x0012ff3c
Level 3: n location 0x0012ff30
Level 4: n location 0x0012ff24
LEVEL 4: n location 0x0012ff24
LEVEL 3: n location 0x0012ff30
LEVEL 2: n location 0x0012ff3c
LEVEL 1: n location 0x0012ff48

递归流程
下面写一个方法，打印出一个十进制整数的二进制表示。例如二进制1101的十进制数为1×2^3 +1×2^2 +0×2^1 +1×2^0 = 13。

#include <stdio.h>
void to_binary(unsigned long n);
int main(void) {
  unsigned long number;
  printf("Enter an integer (q to quit):\n");
  while (scanf("%lu", &number) == 1) {
    printf("Binary equivalent: ");
    to_binary(number);
    putchar('\n');
    printf("Enter an integer (q to quit):\n");
  }
  printf("Done.\n");
  return 0;
}

void to_binary(unsigned long n) { /* recursive function */
  int r;
  r = n % 2;// 即右移>>一下，r即是移出的数
  if (n >= 2)
    to_binary(n / 2);
  putchar(r == 0 ? '0' : '1');
  return;
}

Enter an integer (q to quit):
|13
Binary equivalent: 1101
Enter an integer (q to quit):
|255
Binary equivalent: 11111111
Enter an integer (q to quit):
|1024
Binary equivalent: 10000000000
Enter an integer (q to quit):
|q
done.

斐波纳契数列：

unsigned long Fibonacci(unsigned n) {
    if (n > 2)
        return Fibonacci(n-1) + Fibonacci(n-2);
    else
        return 1;
}