python算法双指针问题：二分查找

Posted 2021-02-07 aguncn

这里要注意的是//向下取整，下次循环时，如果大于查找的数字，start+1。

并且，只能向下取整，如果向上取整。

那么，在比较第一个数时，start = 0 。end = 1。mid = 1。就会进入死循环了。

切记切记。

import math
a_list = [2, 5, 23, 45, 67, 89, 90, 123, 234, 345, 567, 7890, 12132]

guess_no = 67
answer = -1

start, end = 0, len(a_list)

while end - start > 1:
    mid = (end + start) // 2
    print(start, end, mid, a_list[mid])
    if a_list[mid] == guess_no:
        answer = guess_no
        break
    elif a_list[mid] < guess_no:
        start = mid + 1
    else:
        end = mid
else:
    if a_list[start] == guess_no:
        answer = guess_no
        print(‘find it‘)

print(answer)
# ==================上面为书，下面为自己=================
answer = -1

start, end = 0, len(a_list)
while start < end:
    mid = math.floor((start + end) / 2)
    print(start, end, mid, a_list[mid])
    if guess_no < a_list[mid]:
        end = mid
    elif guess_no == a_list[mid]:
        answer = guess_no
        break
    else:
        start = mid + 1
print(answer)

输出：

0 13 6 90
0 6 3 45
4 6 5 89
find it
67
0 13 6 90
0 6 3 45
4 6 5 89
4 5 4 67
67

Process finished with exit code 0