前言

排序算法有很多，这里列出最常用的一些，如选择排序、插入、冒泡等。

稳定性：待排序数据中有相同的数，排序之后相同的数与排序前的前后位置关系不变，则成为稳定排序算法。
比如我们有一组数据2,9,3,4,8,3;按照大小排序之后就是2,3,3,4,8,9;两个3的前后顺序在排序前后保持不变，即稳定。

一、选择排序

代码：

  public void selectSort(int[] arr) {
        for (int i = 0; i < arr.length - 1; i++) {
            int min = i;
            for (int j = i + 1; j < arr.length; j++) {
                if (arr[min] > arr[j]) {
                    min = j;
                }
            }
            if (min != i) {
                swap(arr, i, min);
            }
        }
    }
    private void swap(int[] arr, int i, int j) {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }

二、插入排序

代码：

    public void insertSort(int[] arr) {
        for (int i = 1; i < arr.length; i++) {
            int insertNote = arr[i];
            int j = i - 1;//前一个元素的下标
            while (j >= 0 && insertNote < arr[j]) {
                arr[j + 1] = arr[j];
                j--;
            }
            arr[j + 1] = insertNote;
        }
    }

三、冒泡排序

代码：

    public void bubbleSort(int[] arr) {
        for (int i = 0; i < arr.length - 1; i++) {
            boolean swapped = false;
            for (int j = 0; j < arr.length - 1 - i; j++) {
                if (arr[j] > arr[j + 1]) {
                    swap(arr, j, j + 1);
                    swapped = true;
                }
            }
            if (!swapped) {
                break;
            }
        }
    }
    private void swap(int[] arr, int i, int j) {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }

四、快速排序

代码：

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

    private void quick(int[] arr, int low, int high) {
        if (low < high) {
            int pivot = arr[high]; 
            int pivotIndex = low;
            for (int j = low; j < high; j++) {
                if (arr[j] < pivot) {
                    swap(arr, pivotIndex++, j);
                }
            }
            swap(arr, pivotIndex, high);
            quick(arr, low, pivotIndex - 1);
            quick(arr, pivotIndex + 1, high);
        }
    }
    private void swap(int[] arr, int i, int j) {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }

五、归并排序

代码：

  public void mergeSort(int[] arr) {
        mergeSort(arr, 0, arr.length - 1, new int[arr.length]);
    }

    private void mergeSort(int[] array, int lo, int hi, int[] aux) {
        if (lo < hi) {
            int mid = lo + (hi - lo) / 2;
            mergeSort(array, lo, mid, aux);
            mergeSort(array, mid + 1, hi, aux);
            merge(array, lo, mid, hi, aux);
        }
    }

    private void merge(int[] arr, int lo, int mid, int hi, int[] aux) {
        if (hi - lo + 1 >= 0) {
            System.arraycopy(arr, lo, aux, lo, hi - lo + 1);
        }
        int leftStartIndex = lo;
        int rightStartIndex = mid + 1;
        for (int k = lo; k <= hi; k++) {
            if (leftStartIndex > mid) {//如果左边元素没了， 直接将右边的剩余元素都合并到到原数组中
                arr[k] = aux[rightStartIndex++];
            } else if (rightStartIndex > hi) {//如果右边元素没有了，直接将所有左边剩余元素都合并到原数组中
                arr[k] = aux[leftStartIndex++];
            } else if (aux[leftStartIndex] < aux[rightStartIndex]) {//如果左边比右边小，则将左边的元素拷贝到原数组中
                arr[k] = aux[leftStartIndex++];
            } else {
                arr[k] = aux[rightStartIndex++];
            }
        }
    }

六、希尔排序

代码：

  public void shellSort(int[] arr) {
        for (int gap = arr.length / 2; gap > 0; gap /= 2) {
            for (int i = gap; i < arr.length; i++) {
                int insertNote = arr[i];
                int j = i;
                while (j >= gap && insertNote < arr[j - gap]) {
                    arr[j] = arr[j - gap];
                    j -= gap;
                }
                arr[j] = insertNote;
            }
        }
    }

七、堆排序

代码：

  public void heapSort(int[] arr) {
        log.info("TopMax = {}", top(arr, 5, true));
        log.info("TopMin = {}", top(arr, 5, false));
    }

    private int[] top(int[] input, int topN, boolean topMax) {
        int[] result = Arrays.copyOf(input, topN);

        for (int i = 0; i < topN; i++) {
            int currentIndex = i;
            while (currentIndex != 0 && topMax ? result[parentIndex(currentIndex)] > result[currentIndex] : result[parentIndex(currentIndex)] < result[currentIndex]) {
                swap(result, currentIndex, currentIndex = parentIndex(currentIndex));
            }
        }

        for (int i = topN; i < input.length; i++) {
            if (topMax) {//大顶堆
                if (input[i] > result[0]) {
                    result[0] = input[i];
                    int from = 0;
                    while ((leftIndex(from) < topN && result[from] > result[leftIndex(from)])
                            || (rightIndex(from) < topN && result[from] > result[rightIndex(from)])) {
                        if (rightIndex(from) < topN && result[leftIndex(from)] > result[rightIndex(from)]) {
                            swap(result, from, from = rightIndex(from));
                        } else {
                            swap(result, from, from = leftIndex(from));
                        }
                    }
                }
            } else {//小顶堆
                if (input[i] < result[0]) {
                    result[0] = input[i];
                    int from = 0;
                    while ((leftIndex(from) < topN && result[from] < result[leftIndex(from)])
                            || (rightIndex(from) < topN && result[from] < result[rightIndex(from)])) {
                        if (rightIndex(from) < topN && result[leftIndex(from)] < result[rightIndex(from)]) {
                            swap(result, from, from = rightIndex(from));
                        } else {
                            swap(result, from, from = leftIndex(from));
                        }
                    }
                }
            }

        }
        return result;
    }

    private static int parentIndex(int i) {
        return (i - 1) / 2;
    }

    private static int leftIndex(int i) {
        return 2 * i + 1;
    }

    private static int rightIndex(int i) {
        return 2 * i + 2;
    }
    
    private void swap(int[] arr, int i, int j) {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }

总结