數據結構與算法（八）排序（二） - 歸併排序、快速排序、第K大元素

    public int[] mergeSortC(int[] array, int low, int heigh) {


        if (low < heigh) {
            int mid = (low + heigh) / 2;
            mergeSortC(array, low, mid);
            mergeSortC(array, mid + 1, heigh);
            merge(array, low, mid, heigh);
        }
        return array;
    }

    public void merge(int[] array, int low, int mid, int heigh) {

        int[] temp = new int[heigh - low + 1];
        int i = low;
        int j = mid + 1;
        int k = 0;

        while (i <= mid && j <= heigh) {
            if (array[i] < array[j]) {
                temp[k++] = array[i++];
            } else {
                temp[k++] = array[j++];
            }
        }

        while (i <= mid) {
            temp[k++] = array[i++];
        }
        while (j <= heigh) {
            temp[k++] = array[j++];
        }

        for (int n = 0; n < temp.length; n++) {
            array[n + low] = temp[n];
        }

    }






    public void quickSort(int[] array) {
        quickSortC(array, 0, array.length - 1);
    }

    public void quickSortC(int[] array, int leftIndex, int rightIndex) {

        if (leftIndex >= rightIndex) return;

        int left = leftIndex;
        int right = rightIndex;
        int key = array[left];

        while (left < right) {
            while (right > left && array[right] >= key) {
                right--;
            }
            array[left] = array[right];

            while (left < right && array[left] <= key) {
                left++;
            }
            array[right] = array[left];
        }

        array[left] = key;

        quickSortC(array, leftIndex, left - 1);
        quickSortC(array, right + 1, rightIndex);
    }

  //第K大元素的算法思想來源於快排
    public void quickSortK(int[] array, int leftIndex, int rightIndex, int K) {

        if (leftIndex >= rightIndex) return;

        int left = leftIndex;
        int right = rightIndex;
        int key = array[left];

        while (left < right) {
            while (right > left && array[right] >= key) {
                right--;
            }
            array[left] = array[right];

            while (left < right && array[left] <= key) {
                left++;
            }
            array[right] = array[left];
        }
        //經歷過交換後，left放到了它在數組中對應的位置
        array[left] = key;
        
        //這裏是與快排區別的地方，不需要全都排序，只需要在K的方向上進行排序
        if (left + 1 == K) 
            return;
        else if (left + 1 > K) {
            quickSortK(array, leftIndex, left - 1, K);
        } else {
            quickSortK(array, right + 1, rightIndex, K);
        }

        //最後取數組中第k-1個元素就是第K大的元素
    }