Hoare的分区方案golang

Question

Quicksort with Hoare's partitioning

// Hoare's partitioning scheme
func PartitionHoare(arr []int, low, high int) int {
    length := len(arr)

    if length == 0 {
        panic("Array size is 0")
    }

    pivot := arr[low]
    i := low - 1
    j := high + 1

    for {
        for {
            j--
            if arr[j] > pivot {
                break
            }
        }

        for {
            i++
            if arr[i] < pivot {
                break
            }
        }

        if i < j {
            Swap(&arr, i, j)
        } else {
            return j
        }
    }
}

// Sort
func SortHoare(arr []int, low, high int) {
    if low < high {
        p := PartitionHoare(arr, low, high)
        SortHoare(arr, low, p)
        SortHoare(arr, p+1, high)
    }
}


// Swap i <--> j
func Swap(arr *[]int, i, j int) {
    (*arr)[i], (*arr)[j] = (*arr)[j], (*arr)[i]
}

Trying to implement Quicksort using Hoare's partitioning but can't figure out what am I doing wrong. It is stuck in an infinite loop, always runs out of memory

fatal error: runtime: out of memory

Answer 1

You should use non strict inequalities while looking for position of i and j to do the swap. So instead of

        if arr[j] > pivot {
            break
        }

you should have

        if arr[j] >= pivot {
            break
        }

And the same for i. Instead of

        if arr[i] < pivot {
            break
        }

use

        if arr[i] <= pivot {
            break
        }

Also I'm not sure if it's intentional or not, but currently your algorithm sorts in descending order. If you want to sort in ascending order swap the comparitions between i and j. So:

    if arr[j] <= pivot {
        break
    }

and

    if arr[i] >= pivot {
        break
    }