Leetcode 34. Find First and Last Position of Element in Sorted Array

最新推荐文章于 2024-11-08 13:05:39 发布

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本文介绍了如何使用二分查找法找到有序数组中目标元素的第一个和最后一个位置。提供了两种实现方案，分别是修改后的经典二分查找以及分别从前向后和从后向前搜索的方法。详细代码和思路解析帮助理解算法实现。

文章作者：Tyan
博客：noahsnail.com | 优快云 | 简书

1. Description

Find First and Last Position of Element in Sorted Array

2. Solution

解析：最容易想到的就是二分查找，只是要进行一些修改，另一个方法是分别从前往后找以及从后往前找，满足条件就退出。

Version 1

class Solution:
    def searchRange(self, nums, target):
        length = len(nums)
        start = -1
        end = -1
        left = 0
        right = length - 1

        while left <= right:
            mid = (left + right) // 2
            if nums[mid] > target:
                right = mid - 1
            elif nums[mid] < target:
                left = mid + 1
            else:
                if mid == 0 or nums[mid - 1] < target:
                    start = mid
                    break
                right = mid - 1

        left = 0
        right = length - 1
        while left <= right:
            mid = (left + right) // 2
            if nums[mid] > target:
                right = mid - 1
            elif nums[mid] < target:
                left = mid + 1
            else:
                if mid == length - 1 or nums[mid + 1] > target:
                    end = mid
                    break
                left = mid + 1

        return [start, end]

Version 2

class Solution:
    def searchRange(self, nums, target):
        length = len(nums)
        start = -1
        end = -1
        for i in range(length):
            if nums[i] == target:
                start = i
                break
            if nums[i] > target:
                break

        if start == -1:
            return [start, end]

        for i in range(length -1 , -1, -1):
            if nums[i] == target:
                end = i
                break

            if nums[i] < target:
                break

        return [start, end]