python数据结构与算法之搜索

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 False


if __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<alist[mid]:
            last = mid-1
        else:
            first = mid + 1
    return False

二分查找时间复杂度是O(logn)