【python】二分查找

**必须是有序序列才可以使用二分查找。

1. 查找的数组中无重复元素：

方法一：非递归

def find(data,k):
    first = 0
    last = len(data)-1
    while first <= last:
        mid = (first+last)//2
        if k == data[mid]:
            return mid
        elif k < data[mid]:
            last = mid - 1
        else:
            first = mid + 1

if __name__ == '__main__':
    arr = [1,2,3,4,5,7,8]
    print(find(arr,1))

方法二：递归

def find(data,first,last,k):
    mid = (first+last)//2
    if k == data[mid]:
        return mid
    elif k < data[mid]:
        last = mid - 1
    else:
        first = mid + 1
    return find(data,first,last,k)

if __name__ == '__main__':
    arr = [1,2,3,4,5,7,8]
    print(find(arr,0,len(arr)-1,1))

2. 查找的数组中有重复元素：

def Find(data,first,last,k):
    mid = (first + last)//2
    if k == data[mid]:
        if mid == 0:
            return mid
        elif k == data[mid - 1]:
            last = mid - 1
        elif k != data[mid - 1]:
            return mid
    elif k < data[mid]:
        last = mid - 1
    else:
        first = mid + 1
    return Find(data,first,last,k)
        
if __name__ == '__main__':
    arr = [1,2,3,3,4,4,5,7,8]
    print(Find(arr,0,len(arr)-1,5))