二分搜索

def bitsearch(a,t):
    start=0
    end=len(a)-1
    flag=-1
    while(start<=end):
        mid=int((start+end)/2)
        if a[mid]>t:
            end=mid-1
        elif a[mid]<t:
            start=mid+1
        else:
            flag=mid
            break
    return flag