二分查詢，主要是針對排序問題進行查詢

我們先算出陣列的長度

len = sizeof(a) / sizeof(a[0])

我們先設定

int left = 0;
int right = Len - 1;
int mid = (left + right ) / 2;

如果K大於mid，那麼 left = mid + 1;

小於 right = mid -1;

#include <stdio.h>

int search( int key, int a[], int len )
{
	int k;
	//scanf("%d", &k );
	k = 99;
	int left = 0;
	int right = len - 1;
	int ret = -1;
	while ( right >= left )
	{
		int mid = ( left + right ) / 2;
		if ( a[mid] == k )
		{
			ret = mid;
			break;
		} else if ( a[mid] > k )
		{
			right = mid - 1;
		} else {
			left = mid + 1;
		}
	}
	return ret;
}

int main()
{
	int a[] = {1,3,5,7,8,9,11,34,56,65,67,88,99};
	int len = sizeof(a) / sizeof(a[0]);
	int respond = 0;
	respond = search( 7, a, len );
	printf("%d\n",respond);
	return 0;
}
 

那麼如果我們碰到不是按順序排列的陣列，如何辦呢？

我們可以先排序然後再進行二分查詢

找到最大的數，然後和最後一個數進行swap，重複進行就行了。

#include <stdio.h>

int max( int a[], int len )
{
	int maxid = 0;
	for ( int i = 1; i < len; i++ )
	{
		if ( a[i] > a[maxid] )
		{
			maxid = i;
		}
	}
	return maxid;
}

int main()
{
	int a[] = {2,45,6,8,7,23,67,98,24,65,11,22,32};
	int len = sizeof(a)/sizeof(a[0]);

	for ( int i = len - 1; i > 0; i-- )
	{
		int maxid = max( a, i + 1 );
		//swap a[maxid, a[len - 1]
		int t = a[maxid];
		a[maxid] = a[i];
		a[i] = t;
	}

	for ( int i = 0; i < len; i++ )
	{
		printf("%d ", a[i]);
	}
	printf("\n");
	return 0;
}
 

完整的程式碼為

#include <stdio.h>

int max( int a[], int len )
{
	int maxid = 0;
	for ( int i = 1; i < len; i++ )
	{
		if ( a[i] > a[maxid] )
		{
			maxid = i;
		}
	}
	return maxid;
}

int search( int key, int a[], int len )
{
	int k;
	scanf("%d", &k);
	int left = 0;
	int right = len - 1;
	int ret = -1;
	while ( right >= left )
	{
		int mid = ( left + right ) / 2;
		if ( a[mid] == k )
		{
			ret = mid;
			break;
		} else if ( a[mid] > k )
		{
			right = mid - 1;
		} else {
			left = mid + 1;
		}
	}
	return ret;
}

int main()
{
	int a[] = {1,2,3,4,5,66,54,22,43,78,21,23,24,42};
	int len = sizeof(a) / sizeof(a[0]);

	for ( int i = len - 1; i > 0; i-- )
	{
		int maxid = max( a, i + 1 );
		int t = a[maxid];
		a[maxid] = a[i];
		a[i] = t;
	}

	for ( int i = 0; i < len; i++ )
	{
		printf("%d ",a[i] );
	}
	printf("\n");

	int respond = 0;
	int k;
	printf("input you want to know element: \n");
	respond = search(k, a, len );
	printf("the way is: %d\n", respond );

	return 0;
}