leetcode 34. Find First and Last Position of Element in Sorted Array（python）

描述

Given an array of integers nums sorted in non-decreasing order, find the starting and ending position of a given target value. If target is not found in the array, return [-1, -1]. You must write an algorithm with O(log n) runtime complexity.

Example 1:

Input: nums = [5,7,7,8,8,10], target = 8
Output: [3,4]

Example 2:

Input: nums = [5,7,7,8,8,10], target = 6
Output: [-1,-1]

Example 3:

Input: nums = [], target = 0
Output: [-1,-1]

Note:

0 <= nums.length <= 10^5
-10^9 <= nums[i] <= 10^9
nums is a non-decreasing array.
-10^9 <= target <= 10^9

解析

根据题意，给定一个按非降序排序的整数数组 nums ，找到给定目标值的开始和结束位置。如果在数组中找不到目标，则返回 [-1, -1] 。要求必须编写一个具有 O(log n) 运行时复杂度的算法。

这很明显就是让我们用二分法去找目标值，我们先通过二分法找出目标值的位置，然后分别向左找开始位置，然后向右再找结束位置。

时间复杂度为 O(logN) ，空间复杂度为 O(1) 。

解答

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

运行结果

Runtime: 106 ms, faster than 37.82% of Python online submissions for Find First and Last Position of Element in Sorted Array.
Memory Usage: 14.4 MB, less than 95.12% of Python online submissions for Find First and Last Position of Element in Sorted Array.

原题链接

https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array/

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