二分查找，需要先排序
def binary_search(li,val):
left = 0
right = len(li)-1
while left <= right:
mid = (left+right) // 2
if li[mid] == val:
return mid
elif li[mid] > val:
right = mid -1
else:
left = mid + 1
return None
li = [1,2,5,3,7]
li.sort()
print("li=",li)
ix = binary_search(li,3)
print("ix=",ix)
