Top K Frequent Elements - Python Leetcode Solution

 Given a non-empty array of integers, return the k most frequent elements.

Example 1:

Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]

Example 2:

Input: nums = [1], k = 1
Output: [1]

Note:

• You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
• Your algorithm's time complexity must be better than O(n log n), where n is the array's size.
• It's guaranteed that the answer is unique, in other words the set of the top k frequent elements is unique.
• You can return the answer in any order.

class Solution:
    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
        count = collections.Counter(nums)
        heap = [(-freq, word) for word, freq in count.items()]
        heapq.heapify(heap)
        return [heapq.heappop(heap)[1] for _ in range(k)]
        

TC: O(nlogn) SC: O(n)

class Solution:
    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
        count = collections.Counter(nums)
        buckets = [[] for _ in range(max(count.values())+1)]
        for i,freq in count.items(): buckets[freq].append(i) 
        flat_list = [item for bucket in buckets for item in bucket]
        return flat_list[::-1][:k]

TC: O(n) SC: O(1)

Sort Characters By Frequency - Python Leetcode Solution

 Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:
"tree"

Output:
"eert"

Explanation:
'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

Example 2:

Input:
"cccaaa"

Output:
"cccaaa"

Explanation:
Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.
Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

Input:
"Aabb"

Output:
"bbAa"

Explanation:
"bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.

class Solution:
    def frequencySort(self, s: str) -> str:   
        counts = collections.Counter(s)
        print(counts)
        pq = []
        for t,freq in counts.items():
            heapq.heappush(pq,(-1*freq,t))
        string_builder = []
        while pq:
            freq,t = heapq.heappop(pq)
            string_builder.append(-1* freq * t)
        return ''.join(string_builder)

TC: O(nlogn) SC: O(n)

class Solution:
    def frequencySort(self, s: str) -> str:   
        if not s:
            return ""
        counts = collections.Counter(s)
        maxf = max(counts.values())
        buckets = [[] for _ in range(maxf+1)]
        for c,freq in counts.items():
            buckets[freq].append(c)
            
        string_builder = []
        for i in range(len(buckets)-1,0,-1):
            for c in buckets[i]:
                string_builder.append(c*i)
        return "".join(string_builder)

O(N) O(N)

Balanced Binary Tree - Leetcode Python Solution

 Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as:

a binary tree in which the left and right subtrees of every node differ in height by no more than 1.

 

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4

Return false.

   # Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def isBalanced(self, root: TreeNode) -> bool:
        def check(root):
            if not root:
                return 0
            left = check(root.left)
            right = check(root.right)
            if left == -1 or right == -1 or abs(left-right) > 1:
                return -1
            return 1+max(left,right)
        return check(root) != -1

TC: O(n) SC:O(n)

Linked List Components - Python Solution

 We are given head, the head node of a linked list containing unique integer values.

We are also given the list G, a subset of the values in the linked list.

Return the number of connected components in G, where two values are connected if they appear consecutively in the linked list.

Example 1:

Input: 
head: 0->1->2->3
G = [0, 1, 3]
Output: 2
Explanation: 
0 and 1 are connected, so [0, 1] and [3] are the two connected components.

Example 2:

Input: 
head: 0->1->2->3->4
G = [0, 3, 1, 4]
Output: 2
Explanation: 
0 and 1 are connected, 3 and 4 are connected, so [0, 1] and [3, 4] are the two connected components.

Note:

• If N is the length of the linked list given by head, 1 <= N <= 10000.
• The value of each node in the linked list will be in the range [0, N - 1].
• 1 <= G.length <= 10000.
• G is a subset of all values in the linked list.

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def numComponents(self, head: ListNode, G: List[int]) -> int:
        Gset = set(G)
        cur = head
        noOfcomponents = 0
        while cur:
            if cur.val in Gset and (not cur.next or cur.next.val not in Gset):
                noOfcomponents+=1
                cur = cur.next
            else:
                cur = cur.next
        return noOfcomponents
        

TC: O(n+G) SC:O(G)

Arithmetic Slices - Python Solution (Leetcode)

 A sequence of numbers is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.

For example, these are arithmetic sequences:

1, 3, 5, 7, 9
7, 7, 7, 7
3, -1, -5, -9

The following sequence is not arithmetic.

1, 1, 2, 5, 7

 

A zero-indexed array A consisting of N numbers is given. A slice of that array is any pair of integers (P, Q) such that 0 <= P < Q < N.

A slice (P, Q) of the array A is called arithmetic if the sequence:
A[P], A[P + 1], ..., A[Q - 1], A[Q] is arithmetic. In particular, this means that P + 1 < Q.

The function should return the number of arithmetic slices in the array A.

 

Example:

A = [1, 2, 3, 4]

return: 3, for 3 arithmetic slices in A: [1, 2, 3], [2, 3, 4] and [1, 2, 3, 4] itself.

class Solution:
    def numberOfArithmeticSlices(self, A: List[int]) -> int:
        dp = total = 0
        for i in range(2,len(A)):
            if A[i]-A[i-1] == A[i-1]-A[i-2]:
                dp+=1
                total+=dp
            else:
                dp = 0
        return total
        

TC: O(n) SC: O(1)

Group Shifted Strings - Python Solution Leetcode

 Given a string, we can "shift" each of its letter to its successive letter, for example: "abc" -> "bcd". We can keep "shifting" which forms the sequence:

"abc" -> "bcd" -> ... -> "xyz"

Given a list of non-empty strings which contains only lowercase alphabets, group all strings that belong to the same shifting sequence.

Example:

Input: ["abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"],
Output: 
[
  ["abc","bcd","xyz"],
  ["az","ba"],
  ["acef"],
  ["a","z"]
]

class Solution:
    def groupStrings(self, strings: List[str]) -> List[List[str]]:
        groups = collections.defaultdict(list)
        for s in strings:
            key = ()
            for i in range(len(s)-1):
                key += ((26+ ord(s[i+1]) - ord(s[i])) % 26,)
            groups[key] = groups.get(key,[])+[s]

        return groups.values()
                         

TC:O(no of strings * longest string length) SC: O(n) Hashmap storage.

Flip Equivalent Binary Trees - Python Solution Leetcode

 For a binary tree T, we can define a flip operation as follows: choose any node, and swap the left and right child subtrees.

A binary tree X is flip equivalent to a binary tree Y if and only if we can make X equal to Y after some number of flip operations.

Given the roots of two binary trees root1 and root2, return true if the two trees are flip equivelent or false otherwise.

 

Example 1:

Input: root1 = [1,2,3,4,5,6,null,null,null,7,8], root2 = [1,3,2,null,6,4,5,null,null,null,null,8,7]
Output: true
Explanation: We flipped at nodes with values 1, 3, and 5.

Example 2:

Input: root1 = [], root2 = []
Output: true

Example 3:

Input: root1 = [], root2 = [1]
Output: false

Example 4:

Input: root1 = [0,null,1], root2 = []
Output: false

Example 5:

Input: root1 = [0,null,1], root2 = [0,1]
Output: true

Solution #1

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def flipEquiv(self, root1: TreeNode, root2: TreeNode) -> bool:
        if root1 is root2:
            return True
        if not root1 or not root2 or root1.val != root2.val:
            return False
        return self.flipEquiv(root1.left,root2.left) and self.flipEquiv(root1.right,root2.right) or self.flipEquiv(root1.left,root2.right) and self.flipEquiv(root1.right,root2.left)
        

Aproach #2

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def flipEquiv(self, root1, root2):
        def dfs(node):
            if node:
                yield node.val
                L = node.left.val if node.left else -1
                R = node.right.val if node.right else -1
                if L < R:
                    yield from dfs(node.left)
                    yield from dfs(node.right)
                else:
                    yield from dfs(node.right)
                    yield from dfs(node.left)
                yield '#'
        return all(x==y for x,y in itertools.zip_longest(dfs(root1),dfs(root2)))

TC: O(min(n1,n2)) SC:O(min(H1,H2)) h1,h2 heights of treees.