26.Remove duplicate from Sorted Array

思路：題目不允許開闢新空間，且陣列排好序。若nums[i]==nums[index-1]，增加i去避免複製。當nums[i]!=nums[index-1]，此時進行復制，且讓index+1，重複操作直至i達到len（nums）

class Solution:
    def removeDuplicates(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)==0:
            return 0
        index = 1
        for i in range(1,len(nums)):
            if nums[i] != nums[index-1]:  #找出與nums[index-1]不相等的陣列下標i
                nums[index]=nums[i]    #不同值賦值給num[index]
                index += 1       #此處index+1便於下次賦值，恰好為不重複陣列長度
        return index
 

80.Remove Depulicates from Sorted Array II

思路：同上，重點在於改成判斷nums[i]與nums[index-2]

class Solution:
    def removeDuplicates(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums)<=2:
            return len(nums)
        index = 2
        for i in range(2,len(nums)):
            if nums[i]!=nums[index-2]:
                nums[index] = nums[i]
                index +=1
        return index
 

思路一：暴力破解

思路二：先排序

思路三：去掉重複元素

class Solution:
    def longestConsecutive(self, nums):
        """
        方法一：暴力破解，遍歷nums，記錄current_nums比其值大的連續數，比較當前最長連續數，返回最大的。
		時間複雜度O(n^3)空間複雜度O(1)
        :type nums: List[int]
        :rtype: int
        """
        longest_streak = 0
        for num in nums:  # 可直接通過數，考慮通過索引
            current_num = num
            count = 1
            while current_num + 1 in nums:
                count += 1
                current_num += 1
            longest_streak = max(longest_streak, count)
        return longest_streak

    def longestConsecutive(self, nums):

    """
    方法二：先排序，判斷當前num[i]值是否與num[i+1]相等，相等則i後移。不相等則判斷num[i]+1是否等於nums[i+1]
    相等則說明連續，count+1，不相等重置count，記錄最長連續數。
    返回最長連續數
    時間複雜度O(nlgn)空間複雜度O(1) or O(n)
    """
        if len(nums) == 0:
            return 0
        nums.sort()
        longest_streak = 1
        count = 1
        for i in range(len(nums) - 1):
            if nums[i + 1] != nums[i]:
                if nums[i + 1] == nums[i] + 1:
                    count += 1
                else:
                    longest_streak = max(longest_streak, count)
                    count = 1
        return max(longest_streak, count)


    def longestConsecutive(self, nums):
    """
    方法三：去掉nums中重複數，找出連續數中最大的那個數,若curretn_num-1仍在set(nums)中，count++，返回最長連續數
    返回最長連續數
    時間複雜度O(n)空間複雜度O(n)
    """
        nums_set = set(nums)
        longest_streak = 0
        for num in nums_set:
            if num + 1 not in nums_set:
                current_num = num
                count = 1
                while current_num - 1 in nums_set:
                    current_num -= 1
                    count += 1
                longest_streak = max(longest_streak, count)
        return longest_streak
    # 參考連結：https://leetcode.com/articles/longest-consecutive-sequence/