定義：二分查找又稱折半查找，優點是比較次數少，查找速度快，平均性能好;其缺點是要求待查表為有序表，且插入刪除困難。因此，折半查找方法適用於不經常變動而查找頻繁的有序列表。首先，假設表中元素是按升序排列，將表中間位置記錄的關鍵字與查找關鍵字比較，如果兩者相等，則查找成功;否則利用中間位置記錄將表分成前、後兩個子表，如果中間位置記錄的關鍵字大於查找關鍵字，則進一步查找前一子表，否則進一步查找後一子表。重復以上過程，直到找到滿足條件的記錄，使查找成功，或直到子表不存在為止，此時查找不成功

#Author wangmengzhu
def binary_search(lst,item):
    low 
 
= 0
    high = len(lst) -1
    while low <= high:
        mid = int((low + high)/2)
        guess = lst[mid]
        if guess == item:
            return mid
        elif guess > item:
            high = mid - 1
        else:
            low = mid +1

print(binary_search(lst = [1,2,3,5,34,5,4,6,67],item = 5))

l = [1,2,5,7,10,31,44,47,56,99,102,130,240]
def search_binary(l,item):
    mid_index = len(l) // 2
    if len(l) == 1:
        if l[0] == item:
            print(‘find it‘)
        else:
            print(‘not exists‘)
    if len(l) > 1:
        if l[mid_index] > item:
            l = l[:mid_index]
            search_binary(l,item)
        
 
elif l[mid_index] < item:
            l = l[mid_index:]
            search_binary(l,item)
        else:
            print(‘find it‘)
search_binary(l,32)

l = [1,2,5,7,10,31,44,47,56,99,102,130,240]
def search_binary(l,item):
    mid_index = len(l) // 2
if len(l) == 1:
        if l[0] == item:
            print(‘find it‘)
        else:
            print(‘not exists‘)
    if len(l) > 1:
        if l[mid_index] > item:
            l = l[:mid_index]
            search_binary(l,item)
        elif l[mid_index] < item:
            l = l[mid_index:]
            search_binary(l,item)
        else:
            print(‘find it‘)
search_binary(l,32)

二分查找---二分法