二分查找的思想是:對一個非空的有序的查找表進行如下比較(以下假設是升序排列):
查找找表的低位 low , 高位 high ,
如果 middle = (low + high) / 2 的值等於key值,則查找成功;
如果 middle的值小於key的值,說明key可能在後半段,low=middle ;
如果 middle的值大於key的值, 說明key可能在前半段, high = low ;
直到low >= high表明查找失敗。
下面是二分查找的Java實現版(非遞歸實現):
public static int binSearch(int[] src, int key) {
if (src == null || src.length == 0)
return -1;
int low = 0, high = src.length, middle;
while (low < high) {
middle = (low + high) >> 1;
if (src[middle] == key)
return middle;
else if (src[middle] < key)
low = middle;
else if (src[middle] > key)
high = middle;
}
return -1;
}
二分搜索的遞歸方法實現:
public static int binSearchRecursive(int[] src, int key) {
if (src == null || src.length == 0)
return -1;
return binSearchRecursive(src, key, 0, src.length);
}
private static int binSearchRecursive(int[] src, int key, int low, int high) {
if (low >= high)
return -1;
int middle = (low + high) >> 1;
if (src[middle] == key)
return middle;
else if (src[middle] < key)
return binSearchRecursive(src, key, middle, high);
else if (src[middle] > key)
return binSearchRecursive(src, key, low, middle);
return -1;
}
測試數據:public static int[] DATA_COLLECTION = {
1, 2, 7, 24, 30, 45, 46, 78, 234, 567, 888, 987, 1000
};
public static void main(String[] args) {
System.out.println(binSearch(DATA_COLLECTION, 888));
System.out.println(binSearchRecursive(DATA_COLLECTION, 888));
}
該算法時間複雜度,對於任意長度的查找表,每次檢查後查找區間都縮小一半,所以O(N)=log n 。
下一篇 塊查找。