常见的排序算法(2)

要点

• 快速排序
• 归并排序

1.快速排序

    
    public static void quickSort(int[] arr, int left, int right) {
        if (left < right) {
            int p = partition(arr, left, right);
            quickSort(arr, 0, p);
            quickSort(arr, p + 1, right);
        }

    }

    /**
     * 来回倒腾
     * @param arr
     * @param low
     * @param high
     * @return
     */
    private static int partition(int[] arr, int low, int high) {
        int left = low;
        int right = high;
        int target = arr[low];
        while (left < right) {
            while (arr[right] > =target && right > left) {
                right--;
            }
                        int temp=arr[left];
            arr[left]=arr[right];
            while (arr[left] < target && left < right) {
                left++;
            }
            arr[right]=temp;
        }
        arr[left]=target;
        return left;
    }

2.归并排序

public static void mergeSort(int[] arr) {
        sort(arr, 2, 5);
    }

    public static void sort(int[] arr, int low, int high) {
        if (low < high) {
            int mid = (low + high) / 2;
            sort(arr, low, mid);
            sort(arr, mid + 1, high);
            // 左右归并
            merge(arr, low, mid, high);
        }
    }

    public static void merge(int[] arr, int left, int mid, int right) {
        int[] tmp = new int[right-left+1];// 辅助数组
        int p1 = left, p2 = mid + 1, k = left;// p1、p2是检测指针，k是存放指针
        while (p1 <= mid && p2 <= right) {
            if (arr[p1] <= arr[p2]) {
                tmp[k++] = arr[p1++];
            } else {
                tmp[k++] = arr[p2++];
            }
        }

        while (p1 <= mid) {
            tmp[k++] = arr[p1++];// 如果第一个序列未检测完，直接将后面所有元素加到合并的序列中
        }
        while (p2 <= right) {
            tmp[k++] = arr[p2++];// 同上
        }

        // 复制回原素组
        for (int i = 0; i <= tmp.length; i++) {
            arr[left+i] = tmp[i];
        }

    }