quicksort.go

func main() {
  nums := []{13,7,4,9}
    quicksort(nums, 0, len(nums)-1)
}

func quicksort(arr []int, lo,hi int) {
    if len(arr) < 2 {return}
    if lo < hi {
        p := partition(arr,lo,hi)
        quicksort(arr,lo,p-1)
        quicksort(arr,p+1,hi)
    }
}

func partition(arr []int, i,j int) int {
    pivot := arr[i]
    lo := i // store initial position of i into a variable for later swap
    for i < j {
        for i < j && arr[i] <= pivot {
            i++
        }
        for j >= i && arr[j] >= pivot {  //IMP: j >= is the key, I was only using > and it was failing in those conditions where i == j. It was then not allowing j to decrement
            j--
        }
        if i < j {
            arr[i],arr[j] = arr[j],arr[i]
        }
    }
    //when j has crossed i, swap the j with pivot and return j as the sorted position of the element
    arr[lo],arr[j]=arr[j],arr[lo]
    return j
}

code tested in leetcode