二分查找

def binSearch(a,m):
    low = 0
    high = len(a)-1
    while(low <=high):
        mid = (low+high)/2
        midval = a[mid]
        if midval <m:
            low = mid+1
        elif midval > m:
            high = mid-1
        else:
            return mid
    return -1