Find First and Last Position of Element in Sorted Array

Posted 2021-02-16 wenqinchao

问题：给定一个有序数组和一个目标值，输出目标值在数组中的起始位置和终止位置，如果目标值不在数组中，则输出[-1,-1]

示例：

输入：nums = [1,2,3,5,5,7] target = 5

输出：[3,4]

输入：nums = [1,5,8,9] target = 7

输出：[-1,-1]

解决思路：二分查找直到找到第一个目标值，再对目标值左右进行二分查找

Python代码：   

class Solution(object):
    def searchRange(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        l = 0
        r = len(nums)-1
        while l <= r :
            mid = (l+r)//2
            if nums[mid] > target:
                r = mid - 1
            elif nums[mid] < target:
                l = mid + 1
            else:
                first,last = mid,mid
                while l <= first:
                    fm = (first+l) // 2
                    if nums[fm] < target:
                        l = fm + 1
                    else:
                        first = fm -1
                while r >= last:
                    rm = (last+r) // 2
                    if nums[rm] > target:
                        r = rm - 1
                    else:
                        last = rm + 1
                return [first+1,last-1]
        return [-1,-1]