摸鱼日记2.1

位运算

剑指 Offer 15. 二进制中 1 的个数

右移 >> 除

class Solution {
public:
  int hammingWeight(uint32_t n) {
    int res = 0;
    while (n != 0) {
      res += 1 & n;
      // 右移
      n >>= 1;
    }
    return res;
  }
};

剑指 Offer 65. 不用加减乘除做加法

先无进位，再进位

class Solution {
public:
  int add(int a, int b) {
    while (b != 0) {
      int c = (unsigned int)(a & b) << 1;
      a ^= b;
      b = c;
    }
    return a;
  }
};

啊啊啊啊，田田，不要啊，不要不理我啊 呜呜呜

invert4.c

#include 
#include 

char *itobs(int, char *);
void show_bstr(const char *);
int invert_end(int num, int bits);

int main() {
  char bin_str[CHAR_BIT * sizeof(int) + 1];

  int number;

  puts("Enter integers and see them in binary.");
  puts("Non-numeric input terminates program.");

  while (scanf("%d", &number) == 1) {
    itobs(number, bin_str);
    printf("%d is n", number);
    show_bstr(bin_str);
    putchar('n');
    number = invert_end(number, 4);
    printf("Inverting the last 4 bits givesn");
    show_bstr(itobs(number, bin_str));
    putchar('n');
  }

  puts("Bye!");

  return 0;
}

char *itobs(int number, char *bin_str) {
  int size = CHAR_BIT * sizeof(int);
  int i;
  for (i = size - 1; i >= 0; --i, number >>= 1) {
    bin_str[i] = (01 & number) + '0';
  }

  return bin_str;
}

void show_bstr(const char *bin_str) {
  int i = 0;
  while (bin_str[i]) {
    putchar(bin_str[i]);
    if (++i % 4 == 0 && bin_str[i])
      putchar(' ');
  }
}

int invert_end(int num, int bits) {
  int mask = 0;
  int bitval = 1;
  while (bits-- > 0) {
    mask |= bitval;
    bitval <<= 1;
  }
  return num ^ mask;
}