1.题目描述
2.思路
方法1（自己写的）：一次二分查找找到等于target的一个元素索引axis，然后向左右延伸找边界。
方法2（灵茶山艾府佬的闭区间二分查找写法）：定义一个lower_bound()函数找到第一个大于等于某数的元素索引，分别对target和(target + 1)调用lower_bound()函数即可。
方法3（对方法2的自主延伸）：两次二分查找，分别找小于等于(target - 1)的元素索引以及大于等于(target + 1)的元素索引。
3.代码（Python3）
方法1：
class Solution:
def searchRange(self, nums: List[int], target: int) -> List[int]:
left, right = 0, len(nums) - 1
axis = -1
while left <= right:
mid = (left + right) // 2
if target < nums[left] or target > nums[right]: break
elif nums[mid] < target < nums[right]: left = mid + 1
elif nums[left] < target < nums[mid]: right = mid - 1
else:
if target == nums[left]: axis = left
elif target == nums[right]: axis = right
else: axis = mid
break
if axis == -1: return [-1, -1]
left_bound, right_bound = axis, axis
lb_found, rb_found = False, False
while not lb_found or not rb_found:
if not lb_found:
if left_bound == 0 or nums[left_bound - 1] != target: lb_found = True
else: left_bound -= 1
if not rb_found:
if right_bound == len(nums) - 1 or nums[right_bound + 1] != target: rb_found = True
else: right_bound += 1
return [left_bound, right_bound]方法2：
class Solution:
def searchRange(self, nums: List[int], target: int) -> List[int]:
def lower_bound(target_num):
left, right = 0, len(nums) - 1
while left <= right:
mid = (left + right) // 2
if nums[mid] >= target_num: right = mid - 1
else: left = mid + 1
return left
start = lower_bound(target)
if start == len(nums) or nums[start] != target: return [-1, -1]
end = lower_bound(target + 1) - 1
return [start, end]方法3：
class Solution:
def searchRange(self, nums: List[int], target: int) -> List[int]:
def lower_bound(target_num):
left, right = 0, len(nums) - 1
while left <= right:
mid = (left + right) // 2
if nums[mid] >= target_num: right = mid - 1
else: left = mid + 1
return left
def higher_bound(target_num):
left, right = 0, len(nums) - 1
while left <= right:
mid = (left + right) // 2
if nums[mid] <= target_num: left = mid + 1
else: right = mid - 1
return right
start = lower_bound(target)
if start == len(nums) or nums[start] != target: return [-1, -1]
end = higher_bound(target)
return [start, end]4.执行情况
方法1：
方法2：
方法3：
5.感想
其实方法3完全就是多此一举，是非常没有必要的。灵神的方法也太妙了。
