查找

二分查找：O（logn）

def b(val,n):
    left = 0　
    right = len(val)-1
    while left <= right:
        mid = (left + right) // 2
        if n == val[mid]:
            return mid
        elif n > val[mid]:
            left = mid+1
        else:
            right = mid-1
    return None