源代码
public static int binarySearch(int[] a, int key) {
return binarySearch0(a, 0, a.length, key);
}
public static int binarySearch(int[] a, int fromIndex, int toIndex,
int key) {
rangeCheck(a.length, fromIndex, toIndex);
return binarySearch0(a, fromIndex, toIndex, key);
}
// Like public version, but without range checks.
private static int binarySearch0(int[] a, int fromIndex, int toIndex,
int key) {
int low = fromIndex;
int high = toIndex - 1;
while (low <= high) {
int mid = (low + high) >>> 1;
int 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.
}
思考
为啥是mid + 1 ,mid – 1就一个下标感觉没差啊。
回答:调试之后再回想,发现没有什么差别,最终收拢到 low == high 的时候都能算出来,不会错过。
自己手打的代码
public class Source1_BinarySearch {
public static int binarySearch(int[] a, int fromIndex, int toIndex, int key) {
int low = fromIndex;
int high = toIndex - 1;
while (low <= high) {
int mid = (low + high) >> 1;
int midValue = a[mid];
// 小于 lo 指向中间,大于hi指向中间,等于直接返回
if (midValue < key)
low = mid + 1;
else if (midValue > key)
high = mid - 1;
else
return mid;
}
return -1;
}
public static int binarySearch_Recursive(int[] a, int fromIndex, int toIndex, int key) {
int low = fromIndex;
int high = toIndex - 1;
int result = -1;
if (low <= high) {
int mid = (low + high) >>> 1;
int midValue = a[mid];
if(midValue < key)
result = binarySearch_Recursive(a,mid +1,high + 1,key);
else if (midValue > key)
result = binarySearch_Recursive(a,low,mid + 1 -1,key);
else
return mid;
}
return result;
}
public static void main(String[] args) {
int[] a = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int index = binarySearch_Recursive(a, 0, a.length, 11);
System.out.println(index);
}
}