搜索，顾名思义就是在一个序列中找出某个元素。

二分查找

二分查找只能作用于有序的顺序表中。

def binary_search(alist, item):    """二分查找,递归版"""    n = len(alist)    if n > 0:        mid = n//2        if alist[mid] == item:            return True        elif item < alist[mid]:            return binary_search(alist[:mid],item)        else:            return binary_search(alist[mid+1:],item)    return Falseif __name__ == '__main__':    li = [17,20,26,44,45]    print(binary_search(li,45))    print(binary_search(li, 55))

def binary_search(alist, item):    """二分查找，非递归"""    n = len(alist)    first = 0    last = n-1    while first<=last:        mid = (first+last)//2        if alist[mid] == item:            return  True        elif item

二分查找时间复杂度是O(logn)