First Occurance in Sorted Array

Iterative

package main

import "fmt"

func main() {
    idx := firstOccu([]int{5, 10, 10, 15, 20, 20, 20}, 20)
    fmt.Println(idx)
}

func firstOccu(nums []int, target int) int {
    low := 0
    high := len(nums) - 1
    for low <= high {
        mid := (low + high) / 2
        if nums[mid] == target {
            if mid == 0 || nums[mid-1] != nums[mid] {
                return mid
            }
            high = mid
        } else if nums[mid] < target {
            low = mid + 1

        } else {
            high = mid - 1

        }
    }

    return -1
}

Recursive

package main

import "fmt"

func main() {
    idx := firstOccuRec([]int{5, 10, 10, 15, 20, 20, 20}, 0, 6, 20) //4
    fmt.Println(idx)
}

func firstOccuRec(nums []int, low, high, target int) int {
    if low < high {
        mid := (low + high) / 2
        if nums[mid] == target {
            if mid != 0 && nums[mid-1] == nums[mid] {
                return firstOccuRec(nums, low, mid, target)
            }
            return mid
        } else if nums[mid] < target {
            return firstOccuRec(nums, mid+1, high, target)
        } else {
            return firstOccuRec(nums, low, mid-1, target)
        }
    }

    return -1
}