python經典演算法之折半查詢

1、折半查詢

def s2(li,key):
    li.sort()
    start=0
    end=len(li)-1
    while start<end:
        mid=(start+end)//2
        if li[mid]==key:
            return mid
        elif li[mid]<key:# 後半段
            start=mid+1
        else:
            end=mid-1
    return -1
li=[1,44,55,8,6,-5]
print(sorted(li))#[-5, 1, 6, 8, 44, 55]
print(s2(li,44))#4
 

2、使用遞迴的方式實現折半查詢

li=[23,-8,26,-2,-6,-18,33]
li.sort()
# print(li)
n=len(li)
def fun(li,key,start,end):
    if li[0]<=key<=li[-1]:
        mid = (start+end)//2
        if li[mid]==key:
            return mid
        if start>end:
            return "查不到"
        elif li[mid]<key:
            return fun(li,key,mid+1,end)
        else:
            return fun(li,key,start,mid-1)
    else:
        return "查不到"
# print(fun(li,23,0,n-1))#4
# print(fun(li,3,0,n-1))#查不到
# print(fun(li,-100,0,n-1))#查不到