1. 24. Swap Nodes in Pairs

    class Solution(object):
    def fourSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        if head == None or head.next == None:
            return head
        dummy = ListNode(0);
        dummy.next = head
        p = dummy
        while p.next and p.next.next:
            tmp = p.next.next
            p.next.next ...
    read more

  2. 22. Generate Parentheses

    class Solution(object):
    def generateParenthesis(self, n):
        """
        :type n: int
        :rtype: List[str]
        """
        mylist=[]
        if n == 0:
            return []
        else:
            if n == 1:
                return ["()"]
            else:
                for item in self.generateParenthesis(n-1):
                    for i in range(0,(n-1)*2):
                        newp="("+item[:i]+")"+item[i:]
                        if newp not in mylist:
                            mylist ...
    read more

  3. 21. Merge Two Sorted Lists

    # Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        res=ListNode(0)
        tmp=res
        while l1 != None and l2 != None:
            if l1.val < l2.val:
                tmp ...
    read more

  4. 19. Remove Nth Node From End of List

    class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None


        res = ListNode(0)
        res.next = head
        tmp = res                    # head 1-5, tmp0-5
        for i in range(0, n):        # double pointer, a.head from 1 to None is 5 steps, forward 2. This means the 1st pointer firstly complete ...
    read more

  5. 18. 4Sum

    class Solution(object):
    def fourSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[List[int]]
        """

        nums.sort()
        mylist = []
        if len(nums) < 4:
            return []
        else:
            for i in range(0,len(nums)-3):
                if i != 0 and nums[i] == nums[i-1]:
                    continue
                for j in range ...
    read more

  6. 17. Letter Combinations of a Phone Number

    class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        map = {"2": ["a", "b", "c"],
               "3": ["d", "e", "f"],
               "4": ["g", "h", "i"],
               "5": ["j", "k", "l"],
               "6": ["m", "n", "o"],
               "7": ["p", "q", "r", "s"],
               "8": ["t", "u", "v"],
               "9": ["w", "x", "y", "z"]}
        if len ...
    read more

  7. 16. 3Sum Closest

    class Solution(object):
    def threeSumClosest(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: int
        """
        nums = sorted(nums)
        ans = sum(nums[:3])
        if len(nums) < 3:
            return 0
        elif list(set(nums)) == [0]:
            return 0
        else:
            for i in range(len(nums)):
                if nums[i] == nums[i - 1 ...
    read more

  8. 15. 3Sum

    class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        # # method 1 brute force
        # mylist=[]
        # if len(nums)<3:
        #     return []
        # else:
        #     for i in range(len(nums)):
        #         for j in range(len(nums[i + 1:])):
        #             if (0-nums[i]-nums[i + 1:][j]) in nums[i ...
    read more

  9. 14. Longest Common Prefix

    class Solution(object):
    def longestCommonPrefix(self, strs):
        """
        :type strs: List[str]
        :rtype: str
        """

        if len(strs) <= 1:
            return strs[0] if len(strs) == 1 else ""
        end, minl = 0, min([len(s) for s in strs])
        while end < minl:
            for i in range(1, len(strs)):
                if strs[i][end] != strs ...
    read more

  10. 13. Roman to Integer

    class Solution(object):
    def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """
        values = [1, 4, 5, 9, 10, 40, 50, 90, 100, 400, 500, 900, 1000]
        numerals = ['I', 'IV', 'V', 'IX', 'X', 'XL', 'L', 'XC', 'C', 'CD', 'D', 'CM', 'M']
        sum = 0
        s = s[::-1]
        while s != "":
            if s[:2 ...
    read more

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