作者: 负雪明烛
id: fuxuemingzhu
个人博客: http://fuxuemingzhu.cn/


题目地址:https://leetcode.com/problems/degree-of-an-array/description/

题目描述

Given a non-empty array of non-negative integers nums, the degree of this array is defined as the maximum frequency of any one of its elements.

Your task is to find the smallest possible length of a (contiguous) subarray of nums, that has the same degree as nums.

Example 1:

Input: [1, 2, 2, 3, 1]
Output: 2 Explanation:
The input array has a degree of 2 because both elements 1 and 2 appear twice.
Of the subarrays that have the same degree:
[1, 2, 2, 3, 1], [1, 2, 2, 3], [2, 2, 3, 1], [1, 2, 2], [2, 2, 3], [2, 2]
The shortest length is 2. So return 2.

Example 2:

Input: [1,2,2,3,1,4,2]
Output: 6

Note:

  1. nums.length will be between 1 and 50,000.
  2. nums[i] will be an integer between 0 and 49,999.

题目大意

数组的度是出现次数最多的数字的出现次数。求一个最短子数组的长度,其度等于数组的度。

解题方法

求出最短相同子数组度的长度

题目大意:

给定非空非负整数数组,数组的度是指元素的最大出现次数。

寻找最大连续区间,使得区间的度与原数组的度相同。

想法很粗暴,直接求出整个数组的degree,然后找出所有的度等于该degree的数,找出最小度的数。

import collections
class Solution(object):
def findShortestSubArray(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
if len(nums) == len(set(nums)):
return 1
counter = collections.Counter(nums)
degree_num = counter.most_common(1)[0]
most_numbers = [num for num in counter if counter[num] == degree_num[1]]
scale = 100000000
for most_number in most_numbers:
appear = [i for i,num in enumerate(nums) if num == most_number]
appear_scale = max(appear) - min(appear) + 1
if appear_scale < scale:
scale = appear_scale
return scale

上面使用了Counter,下面的直接数,速度有一点提高。

import collections
class Solution(object):
def findShortestSubArray(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
nums_set = set(nums)
if len(nums) == len(nums_set):
return 1
degree = max([nums.count(num) for num in nums_set])
most_numbers = [num for num in nums_set if nums.count(num) == degree]
scale = 100000000
for most_number in most_numbers:
appear = [i for i,num in enumerate(nums) if num == most_number]
appear_scale = max(appear) - min(appear) + 1
if appear_scale < scale:
scale = appear_scale
return scale

上面的不够快是因为重复计算了多次的nums.count(num),避免重复计算可以使用字典进行保存。这个方法超出了96.7%的提交。

import collections
class Solution(object):
def findShortestSubArray(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
nums_set = set(nums)
if len(nums) == len(nums_set):
return 1
num_dict = {num:nums.count(num) for num in nums_set}
degree = max(num_dict.values())
most_numbers = [num for num in nums_set if num_dict[num] == degree]
scale = 100000000
for most_number in most_numbers:
appear = [i for i,num in enumerate(nums) if num == most_number]
appear_scale = max(appear) - min(appear) + 1
if appear_scale < scale:
scale = appear_scale
return scale

还能更快吗?可以。把能压缩的列表表达式拆开,这样迭代一次就可以了。最后用了个提前终止,如果scale==degree说明这段子列表里没有其他元素了,一定是最短的。

这个方法超过了99.91%的提交。

import collections
class Solution(object):
def findShortestSubArray(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
nums_set = set(nums)
if len(nums) == len(nums_set):
return 1
num_dict = {}
degree = -1
for num in nums_set:
_count = nums.count(num)
num_dict[num] = _count
if _count > degree:
degree = _count
most_numbers = [num for num in nums_set if num_dict[num] == degree]
scale = 100000000
for most_number in most_numbers:
_min = nums.index(most_number)
for i in xrange(len(nums)-1, -1, -1):
if nums[i] == most_number:
_max = i
break
appear_scale = _max - _min + 1
if appear_scale < scale:
scale = appear_scale
if scale == degree:
break
return scale

使用堆求最大次数和最小长度

二刷的时候,想到其实同时优化两个指标:最大次数和最小长度。所以,直接遍历所有的数字,同时统计它的次数,起始位置和结束位置,然后用一个堆,进行最大次数和最小长度的选择,对应的长度就是最小长度。

class Solution:
def findShortestSubArray(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
count = collections.defaultdict(tuple)
for i, num in enumerate(nums):
if num not in count:
count[num] = (1, i, i)
else:
count[num] = (count[num][0] + 1, count[num][1], i)
heap = [(-times, end - start + 1) for times, start, end in count.values()]
heapq.heapify(heap)
return heapq.heappop(heap)[1]

保存最左边出现位置和最右边出现位置

使用两个字典,保存每个数字出现的最左边和最右边位置,这样的话,我们找到了出现次数等于数组的度的数字,然后看它的长度是不是最小的即可。

class Solution:
def findShortestSubArray(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
left, right = dict(), dict()
count = collections.defaultdict(int)
for i, num in enumerate(nums):
if num not in left:
left[num] = i
right[num] = i
count[num] += 1
degree = max(count.values())
res = float("inf")
for num, c in count.items():
if c == degree:
res = min(res, right[num] - left[num] + 1)
return res

日期

2018 年 1 月 23 日
2018 年 11 月 16 日 —— 又到周五了!

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