Python实现二分查找
2017-10-02 21:05
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二分查找:二分查找又称折半查找,优点是比较次数少,查找速度快,平均性能好;其缺点是要求待查表为有序表,且插入删除困难。因此,折半查找方法适用于不经常变动而查找频繁的有序列表。首先,假设表中元素是按升序排列,将表中间位置记录的关键字与查找关键字比较,如果两者相等,则查找成功;否则利用中间位置记录将表分成前、后两个子表,如果中间位置记录的关键字大于查找关键字,则进一步查找前一子表,否则进一步查找后一子表。重复以上过程,直到找到满足条件的记录,使查找成功,或直到子表不存在为止,此时查找不成功。
# 1.以递归方式实现:
def binary_search(num, list1, left_index=0):
binary_index = len(list1) // 2
if num < list1[binary_index]:
binary_search(num, list1[0:binary_index], left_index)
elif num > list1[binary_index]:
left_index = left_index + binary_index
binary_search(num, list1[binary_index: len(list1) + 1], left_index)
else:
print(left_index + binary_index)
if __name__ == "__main__":
list1 = [-2, -1, 0, 2, 3, 4, 5, 6]
binary_search(0, list1)
# 2.以循环方式实现:
def binary_search(target, list1):
left_index = 0
right_index = len(list1)
while left_index <= right_index:
mid = (left_index + right_index) // 2
if list1[mid] == target:
return mid
elif list1[mid] > target:
right_index = mid - 1
else:
left_index = mid + 1
return -1 # 若没有找到则返回0
if __name__ == "__main__":
list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9]
target_index = binary_search(5, list1)
print(target_index)
# 1.以递归方式实现:
def binary_search(num, list1, left_index=0):
binary_index = len(list1) // 2
if num < list1[binary_index]:
binary_search(num, list1[0:binary_index], left_index)
elif num > list1[binary_index]:
left_index = left_index + binary_index
binary_search(num, list1[binary_index: len(list1) + 1], left_index)
else:
print(left_index + binary_index)
if __name__ == "__main__":
list1 = [-2, -1, 0, 2, 3, 4, 5, 6]
binary_search(0, list1)
# 2.以循环方式实现:
def binary_search(target, list1):
left_index = 0
right_index = len(list1)
while left_index <= right_index:
mid = (left_index + right_index) // 2
if list1[mid] == target:
return mid
elif list1[mid] > target:
right_index = mid - 1
else:
left_index = mid + 1
return -1 # 若没有找到则返回0
if __name__ == "__main__":
list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9]
target_index = binary_search(5, list1)
print(target_index)
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