1.冒泡排序

冒泡排序

public class BubbleSort{
    public static void main(String[] args) {
        int[] numbers=new int[]{1,5,8,2,3,9,4};
        int i,j;
        for(i=0;i<numbers.length-1;i++)
        {
            for(j=0;j<numbers.length-1-i;j++)
            {
                if(numbers[j]>numbers[j+1])
                {
                    int temp=numbers[j];
                    numbers[j]=numbers[j+1];
                    numbers[j+1]=temp;
                }
            }
        }
        System.out.println("排序后的结果是:");
        for(i=0;i<numbers.length;i++)
            System.out.print(numbers[i]+" ");
    }
}

2.快速排序

快速排序

public class QuickSort{
    public static void quickSort(int[] arr, int low, int high) {
        int i, j, temp, t;
        if (low > high) {
            return;
        }
        i = low;
        j = high;
        //temp就是基准位
        temp = arr[low];

        while (i < j) {
            //先看右边，依次往左递减
            while (temp <= arr[j] && i < j) {
                j--;
            }
            //再看左边，依次往右递增
            while (temp >= arr[i] && i < j) {
                i++;
            }
            //如果满足条件则交换
            if (i < j) {
                t = arr[j];
                arr[j] = arr[i];
                arr[i] = t;
            }
        }
        //最后将基准为与i和j相等位置的数字交换
        arr[low] = arr[i];
        arr[i] = temp;
        //递归调用左半数组
        quickSort(arr, low, j - 1);
        //递归调用右半数组
        quickSort(arr, j + 1, high);
    }


    public static void main(String[] args) {
        int[] arr = {10, 7, 2, 4, 7, 62, 3, 4, 2, 1, 8, 9, 19};
        quickSort(arr, 0, arr.length - 1);
        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }
    }
}

3.归并排序

image

public class MergeSort {
    //两路归并算法，两个排好序的子序列合并为一个子序列
    public void merge(int []a,int left,int mid,int right){
        int []tmp=new int[a.length];//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(a[p1]<=a[p2])
                tmp[k++]=a[p1++];
            else
                tmp[k++]=a[p2++];
        }

        while(p1<=mid) tmp[k++]=a[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++]=a[p2++];//同上

        //复制回原素组
        for (int i = left; i <=right; i++) 
            a[i]=tmp[i];
    }

    public void mergeSort(int [] a,int start,int end){
        if(start<end){//当子序列中只有一个元素时结束递归
            int mid=(start+end)/2;//划分子序列
            mergeSort(a, start, mid);//对左侧子序列进行递归排序
            mergeSort(a, mid+1, end);//对右侧子序列进行递归排序
            merge(a, start, mid, end);//合并
        }
    }

    @Test
    public void test(){
        int[] a = { 49, 38, 65, 97, 76, 13, 27, 50 };
        mergeSort(a, 0, a.length-1);
        System.out.println("排序完成的数组：");
        for (int e : a)
            System.out.print(e+" ");
    }
}

4.插入排序

image

public class InsertSort {
    public static void main(String[] args) {
        int[] arr = {12,3,2345,645,756,867,8979,879,83,1,3,6,900,45};
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
        System.out.println();

        insertSort(arr);
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }

    //插入排序
    public static void insertSort(int[] arr) {
        int insertVal = 0;
        int insertIndex = 0;
        for(int i = 1; i < arr.length; i++) {
            insertVal = arr[i];
            insertIndex = i - 1;
            while (insertIndex >= 0 && insertVal < arr[insertIndex]) {
                arr[insertIndex + 1] = arr[insertIndex];
                insertIndex--;
            }
            if(insertIndex + 1 != i) {
                arr[insertIndex + 1] = insertVal;
            }
        }
    }
}

5.选择排序

选择排序

public class SelectionSort {
    public static void main(String[] args) {
        int[] arr = {12,32,23,54,56,867,7,6,80};
        selectionSort(arr);
    }

    public static int[] selectionSort(int[] array) {
        if (array.length == 0 || array.length == 1) { return array;}
        for (int i = 0; i < array.length; i++) {
            int minIndex = i;
            for (int j = i; j < array.length; j++) {
                if (array[j] < array[minIndex])
                    minIndex = j;
            }
            int temp = array[minIndex];
            array[minIndex] = array[i];
            array[i] = temp;
        }
        return array;
    }
}

6.两数之和

//一遍hash
class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (map.containsKey(complement)) {
                return new int[] { map.get(complement), i };
            }
            map.put(nums[i], i);
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}
//暴力破解
class Solution {
    public int[] twoSum(int[] nums, int target) {
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[j] == target - nums[i]) {
                    return new int[] { i, j };
                }
            }
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}