MergeSort，归并排序

package _Sort.Algorithm

/**
 * https://www.geeksforgeeks.org/merge-sort/
 * https://www.cnblogs.com/chengxiao/p/6194356.html
 * best/worst/average Time complexity are O(nlogn), Space complexity is O(n), stable
 * # is Divide and Conquer algorithm
 * Basic idea
 * 1. find the middle point to divide the array into two halves;
 * 2. Call MergeSort for first halves;
 * 3. Call MergeSort for second halves;
 * 4. Merge two halves;
 * */
class MergeSort {

    fun sortArray(array: IntArray): IntArray {
        //create temp array first, avoid create temp in recursion
        val tempArray = IntArray(array.size)
        val left = 0
        val right = array.size - 1
        sort(array, left, right, tempArray)
        //printArray(array)
        return array
    }

    private fun printArray(array: IntArray) {
        for (item in array) {
            print("$item,")
        }
    }

    private fun sort(array: IntArray, left: Int, right: Int, tempArray: IntArray) {
        if (left < right) {
            val mid = (left + right) / 2
            sort(array, left, mid, tempArray)
            sort(array, mid + 1, right, tempArray)
            merge(array, left, mid, right, tempArray)
        }
    }

    private fun merge(array: IntArray, left: Int, mid: Int, right: Int, tempArray: IntArray) {
        var i = left
        var j = mid + 1
        var k = 0
        //merge the sub arrays
        while (i <= mid && j <= right) {
            if (array[i] <= array[j]) {
                tempArray[k] = array[i]
                i++
            } else {
                tempArray[k] = array[j]
                j++
            }
            k++
        }

        //add remaining elements of L[]
        while (i <= mid) {
            tempArray[k] = array[i]
            i++
            k++
        }
        //add remaining elements of R[]
        while (j <= right) {
            tempArray[k] = array[j]
            j++
            k++
        }

        //copy temp into original
        k=0
        for (i in left until right+1) {
            array[i] = tempArray[k++]
        }
    }
}