【《漫画算法》笔记】快速排序

2023-12-15 21:40:39

非递归实现

使用集合栈代替递归的函数栈


    public static void main(String[] args) {
        int[] arr=new int[]{4,4,6,4,3,2,8,1};
//        int[] arr=new int[]{3,2};
//        quickSort1(arr,0,arr.length-1); // recursive, double sides
//        quickSort2(arr,0,arr.length-1);// recursive, one side
        quickSort3(arr,0,arr.length-1);// not recursive,
        System.out.println(Arrays.toString(arr));
    }

    private static void quickSort3(int[] arr, int startIdx, int endIdx) {
        Stack<Map<String,Integer>> quickSortStack=new Stack<Map<String, Integer>>();

        Map<String,Integer> rootParam=new HashMap<>(); // 整个数列的起止下标
        rootParam.put("startIdx",startIdx);
        rootParam.put("endIdx",endIdx);
        quickSortStack.push(rootParam);
        //
        while(!quickSortStack.isEmpty()){
            Map<String,Integer> param=quickSortStack.pop();
            int pivotIdx=partition3(arr,param.get("startIdx"),param.get("endIdx"));
            System.out.println(String.valueOf(param.get("startIdx"))+","+String.valueOf(param.get("endIdx")));

            if(param.get("startIdx")<pivotIdx-1){
                Map<String,Integer> leftParam=new HashMap<>();
                leftParam.put("startIdx",startIdx);
                leftParam.put("endIdx",pivotIdx-1);
                quickSortStack.push(leftParam);
            }
            if(param.get("endIdx")>pivotIdx+1){
                Map<String,Integer> rightParam=new HashMap<>();
                rightParam.put("startIdx",pivotIdx+1);
                rightParam.put("endIdx",endIdx);
                quickSortStack.push(rightParam);
            }

        }


    }

    private static int partition3(int[] arr, int startIdx, int endIdx) {
        // System.out.println(Arrays.toString(arr));
        int pivot=arr[startIdx];
        int mark=startIdx;

        for (int i = startIdx; i <=endIdx ; i++) {
            if(pivot>arr[i]){
                mark++;
                int temp=arr[mark];
                arr[mark]=arr[i];
                arr[i]=temp;
            }
        }
        arr[startIdx]=arr[mark];
        arr[mark]=pivot;
        return mark;

    }

注:这个地方有一个很tricky的点是,if (pivot>arr[i])不能写成if(pivot>=arr[i]) (即便是把for循环修正到从startIdx+1开始,也不行,这仍然会造成无限循环,我目前还在找原因)

单边循环的递归写法

if(pivot>=arr[i]) + for(int i=startIdx+1;....)这个搭配出来的partition()函数本身是对的,见下面的“单边循环”递归版快速排序的代码

  public static void main(String[] args) {
        int[] arr=new int[]{4,4,6,4,3,2,8,1};
//        int[] arr=new int[]{3,2};
//        quickSort1(arr,0,arr.length-1); // recursive, double sides
        quickSort2(arr,0,arr.length-1);// recursive, one side

        System.out.println(Arrays.toString(arr));
    }
    
private static void quickSort2(int[] arr, int startIdx, int endIdx) {
        if(startIdx>=endIdx){
            return;
        }

        int pivotIdx=partition2(arr,startIdx,endIdx);
        quickSort2(arr,startIdx,pivotIdx-1);
        quickSort2(arr,pivotIdx+1,endIdx);

    }

    private static int partition2(int[] arr, int startIdx, int endIdx) {
        int mark=startIdx;
        int pivot=arr[mark];
        for (int i = startIdx+1; i <= endIdx; i++) {
            if(arr[i]<=pivot){
                mark++;
                int tmp=arr[i];
                arr[i]=arr[mark];
                arr[mark]=tmp;
            }
        }

        arr[startIdx]=arr[mark];
        arr[mark]=pivot;
        System.out.println(String.valueOf(startIdx)+","+String.valueOf(endIdx));
        System.out.println(Arrays.toString(arr));
        System.out.println();
        return mark;
    }

双边循环的递归写法

public static void main(String[] args) {
        int[] arr=new int[]{4,4,6,4,3,2,8,1};
//        int[] arr=new int[]{3,2};
        quickSort1(arr,0,arr.length-1); // recursive, double sides
//        quickSort2(arr,0,arr.length-1);// recursive, one side
//        quickSort3(arr,0,arr.length-1);// not recursive,
        System.out.println(Arrays.toString(arr));
    }

 private static void quickSort1(int[] arr, int startIdx, int endIdx) {
        if(startIdx>=endIdx){
            return;
        }

        int pivot=partition1(arr,startIdx,endIdx);
        quickSort1(arr,startIdx,pivot-1);
        quickSort1(arr,pivot+1,endIdx);
    }

    private static int partition1(int[] arr, int startIdx, int endIdx) {
        int right=endIdx;
        int left=startIdx;
        int pivot=arr[startIdx];
        while (right!=left){
            while (right>left&&arr[right]>pivot){
                right--;

            }

            while(left<right&&arr[left]<=pivot){
                left++;
            }

            int temp=arr[right];
            arr[right]=arr[left];
            arr[left]=temp;


        }
        arr[startIdx]=arr[left];
        arr[left]=pivot;

        return left;
    }

文章来源:https://blog.csdn.net/qq_43448491/article/details/135005829
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