leetcode刷题笔记-hashtable

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/Sengo_GWU/article/details/82470496

347. Top K Frequent Elements

class Solution(object):
    def topKFrequent(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: List[int]
        """
        
        d, q = {}, {}
        for n in nums:
            if d.get(n) is None:
                d[n] = 1
            else:
                d[n] += 1
                
        for n, c in d.items():
            if q.get(c) is None:
                q[c] = [n]
            else:
                q[c].append(n)
                
        res = [] 
        for i in xrange(len(nums), 0, -1):
            if k < 0:
                break
            elif q.get(i):
                res += [q[i][s] for s in xrange(min(k, len(q[i])))]
                k -= len(q[i])
        return res

454. 4Sum II 

 第一次解法O（n^2），但是Memory Limit Exceeded

class Solution(object):
    def fourSumCount(self, A, B, C, D):
        """
        :type A: List[int]
        :type B: List[int]
        :type C: List[int]
        :type D: List[int]
        :rtype: int
        """
        n, res = len(A), 0
        mapAB = collections.defaultdict(list)
        
        for i in xrange(n):
            for j in xrange(n):
                Sum = A[i] + B[j]
                mapAB[Sum].append([i, j])
        
        for i in xrange(n):
            for j in xrange(n):
                Sum = C[i] + D[j]
                res += len(mapAB[0-Sum])
        
        return res

稍稍修改了下：

class Solution(object):
    def fourSumCount(self, A, B, C, D):
        """
        :type A: List[int]
        :type B: List[int]
        :type C: List[int]
        :type D: List[int]
        :rtype: int
        """
        n, res = len(A), 0
        mapAB = collections.defaultdict(int)
        
        for i in xrange(n):
            for j in xrange(n):
                Sum = A[i] + B[j]
                mapAB[Sum] += 1
        
        for i in xrange(n):
            for j in xrange(n):
                Sum = C[i] + D[j]
                res += mapAB[0-Sum]
        
        return res

138. Copy List with Random Pointer

# Definition for singly-linked list with a random pointer.
# class RandomListNode(object):
#     def __init__(self, x):
#         self.label = x
#         self.next = None
#         self.random = None

class Solution(object):
    def copyRandomList(self, head):
        """
        :type head: RandomListNode
        :rtype: RandomListNode
        """
        d = dict()
        p1 = p2 = head
        
        while p1:
            d[p1] = RandomListNode(p1.label)
            p1 = p1.next
            
        while p2:
            d[p2].next = d.get(p2.next)
            d[p2].random = d.get(p2.random)
            p2 = p2.next
        return d.get(head)