快速排序初窥

partiton

1.在数组a[]找到一个枢纽元(pivot)，pivot与第一个元素交换

2.a[low]为第二个元素，a[high]为倒数第一个元素

3.当low元素小于pivot,low++,当high元素大于pivot,high++,当i,j都停止自增时，若a[i]在a[j]左侧，则交换元素

4.当a[low]在a[high]右侧，停止

5.当high>first且A[high]>=pivot时，high--

6.当a[high] < pivot,交换，返回high，否则返回first

partition代码

 public int partition(int[] A, int first, int last){
        //确定一个pivot，左边小于，右边大于
        //区间第一个元素为pivot
        int pivot = A[first];
        int low = first + 1;
        int high = last;

        while (low < high){
            if(low < high && A[low] <= pivot){
                low++;
            }
            if(low < high && A[high] > pivot){
                high--;
            }
            if(low < high){
                int temp = A[high];
                A[high] = A[low];
                A[low] = temp;
            }
        }
        while (high > first && A[high] >= pivot){
            //查找结束时A[high]等于主元或查找不执行时只有两个元素
            high--;
        }
        if(pivot > A[high]){
            A[first] = A[high];
            A[high] = pivot;
            return high;
        }
        return first;
    }

整体代码

 public  int[] quickSort(int[] A, int n){
        sort(A, 0, n - 1);
        return A;
    }

    public void sort(int[] A, int first, int last){
        if (first < last){
            int partition = partition(A, first, last);
            sort(A, first, partition - 1);
            sort(A, partition+1, last);
        }
    }

    public int partition(int[] A, int first, int last){
        //确定一个pivot，左边小于，右边大于
        //区间第一个元素为pivot
        int pivot = A[first];
        int low = first + 1;
        int high = last;

        while (low < high){
            if(low < high && A[low] <= pivot){
                low++;
            }
            if(low < high && A[high] > pivot){
                high--;
            }
            if(low < high){
                int temp = A[high];
                A[high] = A[low];
                A[low] = temp;
            }
        }
        while (high > first && A[high] >= pivot){
            //查找结束时A[high]等于主元或查找不执行时只有两个元素
            high--;
        }
        if(pivot > A[high]){
            A[first] = A[high];
            A[high] = pivot;
            return high;
        }
        return first;
    }

emmm,影响性能的主要是partition代码，等慢慢学习后会继续修改