基础算法

排序算法

(1) 快速排序(quick sort)

public static void quickSort(int[] array, int left, int right) {
        if (array == null || array.length == 0) return;
        if (left > right) return;
        int key = array[left];
        int l = left;
        int r = right;
        while (l != r) {
            while (array[r] >= key && l < r) {
                r--;
            }
            while (array[l] <= key && l < r) {
                l++;
            }
            if (l < r) {
                int tmp = array[l];
                array[l] = array[r];
                array[r] = tmp;
            }
        }
        array[left] = array[l];
        array[l] = key;
        quickSort1(array, left, l - 1);
        quickSort1(array, l + 1, right);
    }
}

(2) 冒泡排序

public static void bubbleSort(int[] array){
        int i,j,tmp;
        boolean flag;
        for (i=0;i< array.length-1;i++){
            for (j=0,flag=true;j<array.length-1-i;j++){
                if (array[j]>array[j+1]){
                    tmp=array[j];
                    array[j]=array[j+1];
                    array[j+1]=tmp;
                    flag=false;
                }
                if (flag==true) continue;
            }
        }
    }

(3) 选择排序

public static void selectSort(int[] array) {
        int i, j, min, tmp;
        for (i = 0; i < array.length - 1; i++) {
            for (min = i, j = i + 1; j < array.length; j++) {
                if (array[min] > array[j]) {
                    min = j;
                }
            }
            tmp = array[i];
            array[i] = array[min];
            array[min] = tmp;
        }
    }

(4) 插入排序

public static void insertSort(int[] array) {
        int tmp, i, j;
        for (i = 1; i < 10; i++) {
            if (array[i] < array[i - 1]) {
                tmp = array[i];
                for (j = i - 1; j >= 0 && array[j] > tmp; j--) {
                    array[j + 1] = array[j];
                }
                array[j + 1] = tmp;
            }
        }
    }

(5) 堆排序

public static void swap(int[] array, int node1, int node2) {
        int tmp;
        tmp = array[node1];
        array[node1] = array[node2];
        array[node2] = tmp;
    }
    
    public static void heapify(int[] tree, int n, int i) {
        if (i >= n) {
            return;
        }
        int c1 = i * 2 + 1;
        int c2 = i * 2 + 2;
        int max = i;
        if (c1 < n && tree[c1] > tree[max]) {
            max = c1;
        }
        if (c2 < n && tree[c2] > tree[max]) {
            max = c2;
        }
        if (max != i) {
            swap(tree, max, i);
            heapify(tree, n, max);
        }
    }

    public static void build_heap(int[] tree, int n) {
        int last_node = n - 1;
        int parent = (last_node - 1) / 2;
        int i;
        for (i = parent; i >= 0; i--) {
            heapify(tree, n, i);
        }
    }

    public static void heapSort(int[] tree) {
        int n = tree.length;
        build_heap(tree, n);
        int i;
        for (i = n - 1; i >= 0; i--) {
            swap(tree, i, 0);
            heapify(tree, i, 0);
        }
    }

(6) 归并排序

讲解视频：https://www.bilibili.com/video/BV1ZC4y1872h?p=2&spm_id_from=pageDriver

public class MergeSortDemo {
    public static void mergeSort(int[] array, int low, int high) {
        if (low == high) {
            return;
        }
        int mid = (low + high) >>> 1;
        mergeSort(array, low, mid);
        mergeSort(array, mid + 1, high);
        merge(array, low, mid, high);//合并
    }

    private static void merge(int[] array, int low, int mid, int high) {
        int s1 = low;
        int s2 = mid + 1;
        int[] tmp = new int[high - low + 1];//临时数组
        int i = 0; //临时数组tmp的下标
        while (s1 <= mid && s2 <= high) {//说明有数据
            if (array[s1] < array[s2]) {
                tmp[i++] = array[s1++];
            } else {
                tmp[i++] = array[s2++];
            }
        }
        while (s1 <= mid) {
            tmp[i++] = array[s1++];
        }
        while (s2 <= high) {
            tmp[i++] = array[s2++];
        }
        System.out.println(tmp.length);//2 2 4 2 2 4
        for (int j = 0; j < tmp.length; j++) {//复制数组，需注意起始下标加上low，否则每次都是从0号下标开始
            array[j + low] = tmp[j];
        }
    }

    public static void main(String[] args) {
        int[] array = {3, 6, 1, 4, 8, 0, 2, 5};
        mergeSort(array, 0, array.length - 1);
        System.out.println(Arrays.toString(array));
    }
}

复杂度一览