二分查找也称折半查找（Binary Search），它是一种效率较高的查找方法。但是，折半查找要求线性表必须采用顺序存储结构，而且表中元素按关键字有序排列。
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通常在查找数组的时候，会发现一个一个查找很浪费资源，因此通过二分查找法实现 。
算法要求：
1.必须采用顺序存储结构。
2.必须按关键字大小有序排列。
首先，假设表中元素是按升序排列，将表中间位置记录的关键字与查找关键字比较，如果两者相等，则查找成功；否则利用中间位置记录将表分成前、后两个子表，如果中间位置记录的关键字大于查找关键字，则进一步查找前一子表，否则进一步查找后一子表。重复以上过程，直到找到满足条件的记录，使查找成功，或直到子表不存在为止，此时查找不成功。
（上面是思维导图）
#include
#include
//while、do while 、goto、for、递归
void digui(int shang,int xia,int zhong,int num)
{
zhong = (shang + xia) / 2;
if (shang <= xia)
{
if (num == zhong)
{
printf("递归:find.%d\n", zhong);
return;
}
else if (num < zhong)
{
xia = zhong - 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
else
{
shang = zhong + 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
}
return;
}
int main()
{
int a[1024];
for (int i = 0; i<1024; i++) {
a[i] = i;
//printf("%d\n",a[i]);
}
int shang = 0;
int xia = 1023;
int zhong = (shang + xia) / 2;
int num = 102;
while (shang <= xia)//一、while
{
zhong = (shang + xia) / 2;
if (num == zhong)
{
printf("while:find.%d\n", zhong);
break;
}
else if (num < zhong)
{
xia = zhong - 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
else
{
shang = zhong + 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
}
printf("==================\n");
printf("==================\n");
printf("==================\n");
shang = 0;
xia = 1023;
zhong = (shang + xia) / 2;
num = 102;
do {//二、do while
zhong = (shang + xia) / 2;
if (num == zhong)
{
printf("do while:find.%d\n", zhong);
break;
}
else if (num < zhong)
{
xia = zhong - 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
else
{
shang = zhong + 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
} while (xia>=shang);
printf("==================\n");
printf("==================\n");
printf("==================\n");
shang = 0;
xia = 1023;
zhong = (shang + xia) / 2;
num = 988;
AAA://三、goto
zhong = (xia + shang) / 2;
if (num == zhong)
{
printf("goto:find.%d\n", zhong);
goto BBB;
}
else if (num > zhong)
{
shang = zhong + 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
else
{
xia = zhong - 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
if (shang<=xia)
{
goto AAA;
}
BBB:
printf("==================\n");
printf("==================\n");
printf("==================\n");
shang = 0;
xia = 1023;
zhong = (shang + xia) / 2;
num = 288;
for (int i; shang <= xia ;zhong = (shang + xia) / 2)
{
if (num == zhong)
{
printf("for:find.%d\n", zhong);
break;
}
else if (num < zhong)
{
xia = zhong - 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
else
{
shang = zhong + 1;
printf("shang.%d,zhong.%d,xia.%d \n", shang, zhong, xia);
}
}
printf("==================\n");
printf("==================\n");
printf("==================\n");
shang = 0;
xia = 1023;
zhong = (shang + xia) / 2;
num = 288;
digui(shang,xia,zhong,num); //递归
getchar();
getchar();
Sleep(5000);
return 0;
} 
