基本排序看这篇就够了

基本排序

时间复杂度都是O(n^2)

冒泡排序

    public static int[] bubbleSort(int[] array) {
        // Write your code here.
        for (int i = 0; i < array.length - 1; i++) {
            for (int j = 0; j < array.length - 1; j++) {
                if (array[j] > array[j + 1]) {
                    swap(array, j, j + 1);
                }
            }
        }
        return array;
    }

插入排序

    public static int[] insertionSort(int[] array) {
        // Write your code here.
        for (int i = 1; i < array.length; i++) {
            for (int j = 0; j < i; j++) {
                if (array[i] < array[j]) {
                    int temp = array[i];
                    for (int k = i-1; k >= j; k--) {
                        array[k+1] = array[k];
                    }
                    array[j] = temp;
                    break;
                }
            }
        }
        return array;
    }

选择排序

    public static int[] selectionSort(int[] array) {
        // Write your code here.
        for (int i = 0; i < array.length; i++) {
            int min = array[i];
            int mini = i;
            for (int j = i+1; j < array.length; j++) {
                if (array[j] < min) {
                    min = array[j];
                    mini = j;
                }
            }
            swap(array, i, mini);
        }
        return array;
    }

进阶排序

快速排序

时间复杂度O(nlogn)，对于相对有序的数组可能会退化到O(n^2)

    public static int[] quickSort(int[] array) {
        // Write your code here.
        qSort(array, 0, array.length - 1);
        return array;
    }

    public static void qSort(int[] array, int left, int right) {
        if (left < right) {
            int pivot = partition(array, left, right);
            qSort(array, left, pivot - 1);
            qSort(array, pivot + 1, right);
        }
    }

    public static int partition(int[] array, int left, int right) {
        int pivot = (right - left) / 2 + left;
        while (left < right) {
            while (array[left] <= array[pivot] && left < pivot) {
                left++;
            }
            swap(array, left, pivot);
            pivot = left;
            while (array[right] >= array[pivot] && pivot < right) {
                right--;
            }
            swap(array, right, pivot);
            pivot = right;
        }
        return pivot;
    }

快排变形寻找第K大的数

    public int findKth(int[] a, int n, int K) {
        // write code here
        qsort(a, 0, n-1, K);
        return a[K - 1];
    }

    public static void qsort(int[] a, int start, int end, int k) {
        if (start < end) {
            int pivot = partition(a, start, end);
            if (pivot == k - 1) {
                return;
            }
            if (pivot > k - 1) {
                qsort(a, start, pivot - 1, k);
            }
            if (pivot < k - 1) {
                qsort(a, pivot + 1, end, k);
            }
        }

    }

    public static int partition(int[] a, int start, int end) {
        int v = a[start];
        int index = start + 1;
        for (int i = index; i <= end; i++) {
            if (a[i] > v) {
                swap(a, i, index);
                index++;
            }
        }
        swap(a, start, index - 1);
        return index - 1;
    }

归并排序

时间复杂度O(nlogn)

    public static int[] mergeSort(int[] array) {
        // Write your code here.
        mSort(array, 0, array.length - 1);
        return array;
    }

    public static void mSort(int[] array, int left, int right) {
        int mid = (right - left) / 2 + left;
        if (left < right) {
            // attention: must be mid + 1
            mSort(array, left, mid);
            mSort(array, mid + 1, right);
            merge(array, left, mid, right);
        }
    }

    public static void merge(int[] array, int left, int mid, int right) {
        int[] tempArray = new int[right - left + 1];
        int index = 0;
        int leftp = left;
        int rightp = mid + 1;
        while (leftp <= mid && rightp <= right) {
            if (array[leftp] <= array[rightp]) {
                tempArray[index++] = array[leftp++];
            }
            if (array[leftp] > array[rightp]) {
                tempArray[index++] = array[rightp++];
            }
        }
        while (leftp <= mid) {
            tempArray[index++] = array[leftp++];
        }
        while (rightp <= right) {
            tempArray[index++] = array[rightp++];
        }
        System.arraycopy(tempArray, 0, array, left, right-left+1);
    }

堆排序

堆排序时间复杂度O(nlogn)
堆排序首先要建堆，从小到大排序就要建立大根堆，然后进行下沉操作

    public static int[] heapSort(int[] array) {
        // Write your code here.
        buildMaxHeap(array);
        for (int endIdx = array.length - 1; endIdx > 0; endIdx--) {
            swap(array, 0, endIdx);
            siftDown(0, endIdx - 1, array);
        }
        return array;
    }

    public static void buildMaxHeap(int[] array) {
        int firstParentIdx = (array.length - 2) / 2;
        for (int currentIdx = firstParentIdx; currentIdx >=0; currentIdx--) {
            siftDown(currentIdx, array.length - 1, array);
        }
    }

    public static void siftDown(int currentIdx, int endIdx, int[] array) {
        int childOne = currentIdx * 2 + 1;
        while (childOne <= endIdx) {
            int childTwo = childOne + 1 <= endIdx ? childOne + 1 : -1;
            int swapToIdx = childOne;
            if (childTwo != -1 && array[childTwo] > array[childOne]) {
                swapToIdx = childTwo;
            }
            if (array[swapToIdx] > array[currentIdx]) {
                swap(array, swapToIdx, currentIdx);
                currentIdx = swapToIdx;
                childOne = currentIdx * 2 + 1;
            } else {
                return;
            }
        }
    }

堆还有一个上浮操作，主要用在插入一个节点的时候，堆插入节点都为最后一个位置。堆删除一个节点只能删除第一个节点。

public void siftUp(int currentIdx, List<Integer> heap) {
	// Write your code here.
    int parentIndex = (currentIdx - 1) / 2;
	while (currentIdx > 0 && heap.get(currentIdx) < heap.get(parentIndex)) {
		swap(currentIdx, parentIndex, heap);
        currentIdx = parentIndex;
        parentIndex = (currentIdx - 1) / 2;
	}
}

本文地址：https://blog.csdn.net/qq_41667282/article/details/109566104