剑指Offer数据结构之Heap[Python版]

前段时间腾讯视频面遇到的题目，第一个方法用冒泡实现时间复杂度太高，又用堆优化了一遍，还可以进一步优化，下面总结一下

面试题029 最小的K个数

题目描述：输入n个整数，找出其中最小的K个数。例如输入4,5,1,6,2,7,3,8这8个数字，则最小的4个数字是1,2,3,4,。
解题思路：排序
代码 速度很快

class Solution(object):
    def smallestK(self, arr, k):
        """
        :type arr: List[int]
        :type k: int
        :rtype: List[int]
        """
        if not arr or len(arr)<k:
            return []
        arr.sort()
        return arr[:k]

代码 From leetcode 速度慢

# 标准快排
class Solution(object):
    def smallestK(self, arr, k):
        """
        :type arr: List[int]
        :type k: int
        :rtype: List[int]
        """
        def quick_sort(nums):
            #用2个数组来分别保存跟基准相比大或者小的数
            left,right = [],[]
            if(len(nums) < 2):
                return nums
            else:
                base = nums[0]
                #每一次以数组的第一个值为基准，小于等于的放到左边数组，大于的放到右边数组
                left = [num for num in nums[1:] if num <= base]
                right = [num for num in nums[1:] if num > base]
                #如果刚好左边的小数组序列长度符合要求就可以直接返回了，因为题目没有要求返回顺序
                if(len(left) == k):
                    return left
                else:
                    return quick_sort(left) + [base] + quick_sort(right)
        return quick_sort(arr)[:k]

# 归并排序
class Solution:
    def GetLeastNumbers_Solution(self, tinput, k):
        # write code here
        def merge_sort(lst):
            if len(lst) <= 1:
                return lst
            mid = len(lst) // 2
            left = merge_sort(lst[: mid])
            right = merge_sort(lst[mid:])
            return merge(left, right)
        def merge(left, right):
            l, r, res = 0, 0, []
            while l < len(left) and r < len(right):
                if left[l] <= right[r]:
                    res.append(left[l])
                    l += 1
                else:
                    res.append(right[r])
                    r += 1
            res += left[l:]
            res += right[r:]
            return res
        if tinput == [] or k > len(tinput):
            return []
        tinput = merge_sort(tinput)
        return tinput[: k]

# 堆排序
class Solution:
    def GetLeastNumbers_Solution(self, tinput, k):
        # write code here
        def siftup(lst, temp, begin, end):
            if lst == []:
                return []
            i, j = begin, begin * 2 + 1
            while j < end:
                if j + 1 < end and lst[j + 1] > lst[j]:
                    j += 1
                elif temp > lst[j]:
                    break
                else:
                    lst[i] = lst[j]
                    i, j = j, 2 * j + 1
            lst[i] = temp
 
        def heap_sort(lst):
            if lst == []:
                return []
            end = len(lst)
            for i in range((end // 2) - 1, -1, -1):
                siftup(lst, lst[i], i, end)
            for i in range(end - 1, 0, -1):
                temp = lst[i]
                lst[i] = lst[0]
                siftup(lst, temp, 0, i)
            return lst
 
        if tinput == [] or k > len(tinput):
            return []
        tinput = heap_sort(tinput)
        return tinput[: k]

# 冒泡排序
class Solution:
    def GetLeastNumbers_Solution(self, tinput, k):
        # write code here
        def bubble_sort(lst):
            if lst == []:
                return []
            for i in range(len(lst)):
                for j in range(1, len(lst) - i):
                    if lst[j-1] > lst[j]:
                        lst[j-1], lst[j] = lst[j], lst[j-1]
            return lst
 
        if tinput == [] or k > len(tinput):
            return []
        tinput = bubble_sort(tinput)
        return tinput[: k]

# 直接选择
class Solution:
    def GetLeastNumbers_Solution(self, tinput, k):
        # write code here
        def select_sort(lst):
            if lst == []:
                return []
            for i in range(len(lst)-1):
                smallest = i
                for j in range(i, len(lst)):
                    if lst[j] < lst[smallest]:
                        smallest = j
                lst[i], lst[smallest] = lst[smallest], lst[i]
 
            return lst
 
        if tinput == [] or k > len(tinput):
            return []
        tinput = select_sort(tinput)
        return tinput[: k]

# 插入排序
class Solution:
    def GetLeastNumbers_Solution(self, tinput, k):
        # write code here
        def Insert_sort(lst):
            if lst == []:
                return []
            for i in range(1, len(lst)):
                temp = lst[i]
                j = i
                while j > 0 and temp < lst[j - 1]:
                    lst[j] = lst[j - 1]
                    j -= 1
                lst[j] = temp
            return lst
 
        if tinput == [] or k > len(tinput):
            return []
        tinput = Insert_sort(tinput)
        return tinput[: k]