第一次上九十 | #二分查找-II#

class Solution:
    def search(self , nums , target ):
        # write code here
        if not nums: return -1
        left, right = 0, len(nums)-1
        mark = False
        while left<=right:
            median = left + (right-left)//2
            if nums[median]>target:right = median-1
            elif nums[median]==target:right = median-1;mark = True
            else:
                left = median+1
        return left if mark else -1

二分法的边界和返回值总是很难确定，有没有什么技巧呀