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# 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

# O(n) O(n)
class Solution(object):
    def copyRandomList(self, head):

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class Solution(object):
  # O(n) O(1)
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """

        if len(prices) < 2:
            return 0

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class Solution(object):
  # O(n*k) O(n*k)
    def maxProfit(self, k, prices):
        """
        :type k: int
        :type prices: List[int]
        :rtype: int
        """
        # corner case
        if not prices or k == 0:

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class Solution(object):
  # O(n) O(1)
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """
        
        # result placeholder
        max_profit = 0

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class Solution(object):
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """
        # O(n) O(1)
        # for each price points, we want to know the lowest price before and 
        # then we can get the max profit
        

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

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class Solution {
    public List<String> topKFrequent(String[] words, int k) {
        // hashmap stores counts of each words
        HashMap<String, Integer> map = new HashMap<>();
        for(String s : words){
            map.put(s, map.getOrDefault(s, 0) + 1);
        }
        // PQ to get top k elements
        PriorityQueue<Map.Entry<String, Integer>> pq = new PriorityQueue<>(
            (a, b) -> a.getValue() == b.getValue() ? b.getKey().compareTo(a.getKey()): a.getValue() - b.getValue()

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class Solution(object):
  # O(n) O(1)
    def moveZeroes(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        i = 0 
        j = 0
        

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# use dictionary
import collections

class Solution(object):
  # O(n)
    def firstUniqChar(self, s):
        """
        :type s: str
        :rtype: int
        """

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class Solution(object):
  # O(nlgn*k) O(n*k)
    def groupAnagrams(self, strs):
        """
        :type strs: List[str]
        :rtype: List[List[str]]
        """
        groups = dict()
        res = []