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

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

Given an array of integers nums sorted in ascending order, find the starting and ending position of a given target value.

Your algorithm's runtime complexity must be in the order of O(log n).

If the target is not found in the array, return [-1, -1].

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]

思路：

在一个有序数组中找到目标的首尾下标，要求时间复杂度为O(logn)，很明显使用二分法来做，与普通二分法不同的是，需要当mid元素目标匹配的时候，判断该移动右指针走，找到左边界，还是移动左指针，找到右边界。所以需要两次二分搜索。

代码：

go：

func searchRange(nums []int, target int) []int {
    var res = []int{-1, -1}
    if nums == nil || len(nums) == 0 {
        return res
    }
    
    // find left
    low, high := 0, len(nums) - 1
    for  ; low + 1 < high; {
        mid := low + (high - low)/2
        if nums[mid] < target {
            low = mid
        } else {
            high = mid
        }
    } 
    if nums[low] == target {
        res[0] = low
    } else if nums[high] == target {
        res[0] = high
    }
    
    if res[0] == -1 {
        return res
    }
    
    // find right
    low, high = 0, len(nums) - 1
    for ; low + 1 < high; {
        mid := low + (high - low)/2
        if nums[mid] <= target {
            low = mid
        } else {
            high = mid
        }
    }
    
    if nums[high] == target {
        res[1] = high
    } else if nums[low] == target {
        res[1] = low
    }
    

    return res
}