Basic Sorting

冒泡、选择、插入

Bubble Sort 冒泡排序

For each pass, we will move left to right swapping adjacent elements as needed. Each pass moves the next largest element into its final position.

以下是它的visualization

https://www.hackerearth.com/zh/practice/algorithms/sorting/bubble-sort/visualize/www.hackerearth.com

简单说，内外两个循环，外循环控制这是第几轮交换（1轮让max在最后，2轮让second largest在倒数第二...），内循环控制相邻位置的交换。

solution 1

def bubble_sort (list):
    for n in range (len(list)-1,0,-1):
        for i in range(n):
            if (list[i]>list[i+1]):
                list[i],list[i+1]=list[i+1],list[i]
    return list

solution 2

Space O(1) (in place)

Time O( $n^{2}$ )

Selection Sort 选择排序

n个元素，做n轮，每轮选出剩余元素最小值，归位。 For each pass, we will move from left to right looking for the next largest(smallest) value. Once that is found, it will be swapped into its final position.

以下是它的visualization

https://www.hackerearth.com/zh/practice/algorithms/sorting/selection-sort/visualize/www.hackerearth.com

先要解决的子问题：找到一个array中最小的数

def find_min(array):
    min_value = array[0]
    for i in range (len(array)):
        if array[i]<min_value:
            min_value=array[i]
    return min_value

然后推广到每一层

solution 1 find_min

class Solution(object):
  def solve(self, array):
    """
    input: int[] array
    return: int[]
    """
    # write your solution here
    if not array or len(array)==0:
      return array

    for i in range (0, len(array)): #也可range(len(array))
      minIdx=i
      for j in range(i, len(array)): #也可i+1
        if array[j]<array[minIdx]:
          minIdx=j
      array[i], array[minIdx] = array[minIdx], array[i]
    
    return array

solution 2 find_max

Space O(1) (in place)

Time O( $n^{2}$ )

3 stacks 实现选择排序 无重复

stack1(store input)

stack2(buffer) global min=

stack3(store output)

Time O( $n^{2}$ )

3 stacks 实现选择排序 有重复

2 stacks 实现选择排序

Insertion Sort 插入排序

Insertion sort is based on the idea that one element from the input elements is consumed in each iteration to find its correct position i.e, the position to which it belongs in a sorted array.

It iterates the input elements by growing the sorted array at each iteration. It compares the current element with the largest value in the sorted array. If the current element is greater, then it leaves the element in its place and moves on to the next element else it finds its correct position in the sorted array and moves it to that position. This is done by shifting all the elements, which are larger than the current element, in the sorted array to one position ahead.

https://www.hackerearth.com/zh/practice/algorithms/sorting/insertion-sort/visualize/www.hackerearth.com

先要解决的子问题：如何在数组中插入一个数，让它能在correct position。

def insert_num(array, n):
    idx=len(array)-1
    array.append(n)
    while idx>=0:
            if array[idx]>array[idx+1]:
                array[idx],array[idx+1]=array[idx+1],array[idx]
            idx-=1
    return array

方法一：

调用insert_num这个help function，实现insertion sort

方法一 O(N)的空间消耗

def insert_sort(array):
    new_array=[]
    for i in range (len(array)):
        insert_num(new_array, array[i])
    return new_array

Space O(n) 在line2中建立了一个空数组，往空数组里加元素，多占了n

Time O( $n^{2}$ )

所以这不是最优解

方法二：

像轮回洗牌似的操作, in place地进行插入排序。这个程序看起来会比上面的更简单，但是tricky的是要提前想明白所定义变量的物理意义。i之前的元素是已经排序好的，k是用来在内层循环中一个个往前找到cur应该处的位置的，在找的同时把元素一个个换过去，最后cur是现在需要排序的对象。

First Tab

def insert_sort (array):
    for i in range (len(array)):
        cur = array[i]
        k = i
        while k>0 and cur<array[k-1]:
                array[k]=array[k+1]
                k-=1
         array[k]=cur   

Second Tab

Space O(1) (in place)

Time O( $n^{2}$ )

方法三：

也可以直接去优化方法一，因为binary search可以简化找位置的操作，让时间复杂度缩减到 $O(log(n))$ , 交换的操作是 $O(n)$, 最后总的时间复杂度依然是$O(log(n)+n)*n=O(n^{2})$

这个有点问题 但我还不知道啥问题= =

def insert_num (array, n):
  idx=len(array)-1
  array.append(n)
  if idx<0:
    return n
  else:
    insert_place=binary_search(array, n) 
    for i in range(insert_place+1, idx):
     array[i+1]=array[i]
    array[insert_place]=n
    return array
    
def binary_search (nums, target):
  left=0
  right=len(nums)-1
  while left<right-1:
      mid=(left+right)/2
      if nums[mid]>target:
        right=mid
      elif nums[mid]<target:
        left=mid
      else:
        return mid
  return left 

array = [5,3,2,1]
new_array=[]
for i in range (len(array)):
  insert_num (new_array, array[i])

print new_array