Day4

21. Merge Two Sorted Lists
    **思路：二路归并，用链表的数据结构
    将链表的表头虚化，然后比较两个排序列表的数值，小的先接到新的链表的Next节点，旧链表往后移。有了新值后，新链表往后移。最后直接连接剩余链表。

# 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
        """
        if l1 == None and l2 == None:
            return None
        if l1 == None:
            return l2
        if l2 == None:
            return l1
        ret = ListNode(0)
        head = ret
        while l1 != None and l2 != None:
            if l1.val <=l2.val:
                ret.next = l1
                l1 = l1.next
            else:
                ret.next = l2
                l2 = l2.next
            ret = ret.next
                
        if l1 == None:
            ret.next = l2
        if l2 == None:
            ret.next = l1
        
        return head.next

26. Remove Duplicates from Sorted Array
    **思路：新数组不能开辟新的空间，那就在原数组基础上，如果两两不等，he原数组一样，如果出现不同，数组就跳过。nums[j]相当于新数组最后一个元素，所以返回的是j+1

class Solution(object):
    def removeDuplicates(self, nums):
        if len(nums)==0:
            return 0
        j=0
        for i in range(1,len(nums)):
            if nums[i]!=nums[j]:
                nums[j+1]=nums[i]
                j=j+1
        return  j+1