我有两个长度为l的数组A和B,一个按升序排序,另一个按降序排序。我需要找到一个索引 k,使得对于区间 [0,l-1] max(A[k], B[k]) 中的所有 k 来说,max(A[k], B[k]) 是最小的。有没有办法在小于 O(l) 的时间内做到这一点?
因为一个数组按升序排序,另一个数组按降序排序,因此您可以利用这种排序性,通过二分搜索在
k
时间内找到最佳 log(l)
。
请参阅下面的 Python 实现:
def find_min_max_index(a: list[int], b: list[int]) -> int:
"""
Finds the index k for which the maximum of a[k] and b[k] is minimal.
This function assumes that list a is sorted in ascending order and list b is sorted
in descending order. It uses a binary search approach to find the optimal index k
efficiently.
Args:
a: The first list, sorted in ascending order.
b: The second list, sorted in descending order.
Returns:
int: The index k such that max(a[k], b[k]) is minimal.
If the function cannot find such an index, it returns -1.
"""
low, high = 0, len(a) - 1
while low < high:
mid = (low + high) // 2
if max(a[mid], b[mid]) > max(a[mid + 1], b[mid + 1]):
low = mid + 1
else:
high = mid
return low
def main() -> None:
print(f'{find_min_max_index(a=[1, 3, 5], b=[3, 2, 1]) = }')
print(f'{find_min_max_index(a=[1, 2, 3], b=[3, 2, 1]) = }')
print(f'{find_min_max_index(a=[2, 4, 8, 16, 32], b=[64, 48, 36, 24, 12]) = }')
if __name__ == '__main__':
main()
输出:
find_min_max_index(a=[1, 3, 5], b=[3, 2, 1]) = 0
find_min_max_index(a=[1, 2, 3], b=[3, 2, 1]) = 1
find_min_max_index(a=[2, 4, 8, 16, 32], b=[64, 48, 36, 24, 12]) = 3