用指针实现高低位倒序，疯了吧

昨晚在微信群看到一个读者发的面试题目，从网上截图出来的，我百思不得其解，题目如图。

幸好，我学过栈

栈，C语言实现

然后我写了个小程序

第一个方法比较笨，当我写完自己的代码后，看到有同学发了自己的代码，我赶紧就发了个红包，一个是为了鼓励大家多讨论问题，一个是为了赞扬这样的行为。

第一个方法就是取出每一个bit，压栈，然后再出栈，你想怎么排序都没有问题。

#include "stdio.h"
#include "stdlib.h"struct List{int data;struct List * next;
};struct Stack{struct List *head;int size;
};struct Stack * StackInit(void)
{struct Stack *stack = NULL;stack =  (struct Stack*)malloc(sizeof(struct Stack));stack->head = (struct List *)malloc(sizeof(struct List));stack->head->next = NULL;stack->size = 0;return stack;
}int StackPush(struct Stack *stack,int data)
{struct List *tmp = (struct List *)malloc(sizeof(struct List));tmp->data = data;tmp->next = stack->head->next;stack->head->next = tmp;stack->size++;//printf("push:%d \n",data);return 0;
}int IsStackEmpty(struct Stack *stack)
{if(stack->head->next == NULL){//printf("stack null\n");return 1;}else{//printf("stack is not null\n");return 0;}
}int StackPop(struct Stack *stack,int *data)
{struct List *tmp = NULL;if(IsStackEmpty(stack))return -1;tmp = stack->head->next;*data = tmp->data;stack->head->next = tmp->next;stack->size--;if(tmp!=NULL)free(tmp);//printf("pop:%d \n",*data);return 0;
}int main(void)
{int i = 0;unsigned char a = 0x71;unsigned char b = 0;struct Stack *stack = NULL;stack = StackInit();printf("a:0x%x\n",a);for(i = 0;i<8;i++){if(a&0x10) StackPush(stack,1);else       StackPush(stack,0);if(a&0x20) StackPush(stack,1);else       StackPush(stack,0);if(a&0x40) StackPush(stack,1);else       StackPush(stack,0);if(a&0x80) StackPush(stack,1);else       StackPush(stack,0);if(a&0x01) StackPush(stack,1);else       StackPush(stack,0);		if(a&0x02) StackPush(stack,1);else       StackPush(stack,0);if(a&0x04) StackPush(stack,1);else       StackPush(stack,0);	if(a&0x08) StackPush(stack,1);else       StackPush(stack,0);					}for(i = 0;i<8;i++){int d = 0;StackPop(stack,&d);if(i == 0)if(d == 1) b |= 0x80;if(i == 1)if(d == 1) b |= 0x40;if(i == 2)if(d == 1) b |= 0x20;if(i == 3)if(d == 1) b |= 0x10;if(i == 4)if(d == 1) b |= 0x08;if(i == 5)if(d == 1) b |= 0x04;if(i == 6)if(d == 1) b |= 0x02;if(i == 7)if(d == 1) b |= 0x01;}printf("\nb:0x%x\n",b);return 0;
}

执行如下：

除了用栈来实现，群里有位同学的答案让我眼前一亮

#include <stdio.h>struct charbits{unsigned char data1 : 4;unsigned char data2 : 4;
};union dataChange{struct charbits charBits;unsigned char data;
}datchange,*p;int main(){unsigned char temp;p = &datchange;datchange.data = 0x34;temp = p->charBits.data1;p->charBits.data1 = p->charBits.data2;p->charBits.data2 = temp;printf("data is %x", datchange.data);return 0;
}

完美演示了位域、共用体、结构体、指针的几个知识点~

测试下位域的作用

#include <stdio.h>struct charbits{unsigned char data1 : 1;unsigned char data2 : 3;
};struct charbits1{unsigned char data1;unsigned char data2;
};int main(){struct charbits a;struct charbits1 b;printf("%d %d\n",sizeof(a),sizeof(b));/*赋值1看看*/a.data1 = 1;printf("%d\n",a.data1);/*赋值3看看*/a.data1 = 3;printf("%d\n",a.data1);/*赋值3看看*/a.data2 = 3;printf("%d\n",a.data2);/*赋值7看看*/a.data2 = 7;printf("%d\n",a.data2);/*赋值8看看*/a.data2 = 8;printf("%d\n",a.data2);return 0;
}

data1只能存 0和1，如果存其他值的话就会溢出变成0。

data2是 3bits ，只能存 0~7，我们存 8 进去，就溢出变成 0。