其实这个题目并不难,但是却是很实用,因为很多同学对于异常条件吃不准的话就会有问题。
看看JDK自带的二分查找吧
/**
* Searches a range of
* the specified array of longs for the specified value using the
* binary search algorithm.
* The range must be sorted (as
* by the {@link #sort(long[], int, int)} method)
* prior to making this call. If it
* is not sorted, the results are undefined. If the range contains
* multiple elements with the specified value, there is no guarantee which
* one will be found.
*
* @param a the array to be searched
* @param fromIndex the index of the first element (inclusive) to be
* searched
* @param toIndex the index of the last element (exclusive) to be searched
* @param key the value to be searched for
* @return index of the search key, if it is contained in the array
* within the specified range;
* otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>. The
* <i>insertion point</i> is defined as the point at which the
* key would be inserted into the array: the index of the first
* element in the range greater than the key,
* or <tt>toIndex</tt> if all
* elements in the range are less than the specified key. Note
* that this guarantees that the return value will be >= 0 if
* and only if the key is found.
* @throws IllegalArgumentException
* if {@code fromIndex > toIndex}
* @throws ArrayIndexOutOfBoundsException
* if {@code fromIndex < 0 or toIndex > a.length}
* @since 1.6
*/
public static int binarySearch(long[] a, int fromIndex, int toIndex,
long key) {
rangeCheck(a.length, fromIndex, toIndex);
return binarySearch0(a, fromIndex, toIndex, key);
}
/**
* Checks that {@code fromIndex} and {@code toIndex} are in
* the range and throws an exception if they aren't.
*/
private static void rangeCheck(int arrayLength, int fromIndex, int toIndex) {
if (fromIndex > toIndex) {
throw new IllegalArgumentException(
"fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")");
}
if (fromIndex < 0) {
throw new ArrayIndexOutOfBoundsException(fromIndex);
}
if (toIndex > arrayLength) {
throw new ArrayIndexOutOfBoundsException(toIndex);
}
}
// Like public version, but without range checks.
private static int binarySearch0(long[] a, int fromIndex, int toIndex,
long key) {
int low = fromIndex;
int high = toIndex - 1;
while (low <= high) {
int mid = (low + high) >>> 1;
long midVal = a[mid];
if (midVal < key)
low = mid + 1;
else if (midVal > key)
high = mid - 1;
else
return mid; // key found
}
return -(low + 1); // key not found.
}
来源:CSDN
作者:自然鸟神
链接:https://blog.csdn.net/chenpeng19910926/article/details/104028078