daily-coding-problem

leet-code-problems

There is a very nice platform for solving different kinds of problems LeetCode. Sometimes I try to solve their problems and publish some solutions here. Definitely encourage you to visit their webpage.

Problem 1

Given a list of numbersand a number k, return whether any two numbers from the list add up to k.
For example, given [10, 15, 3, 7] and k of 17, return true since 10 + 7 is 17.
Bonus: Can you do this in one pass?

Solution #1

Problem 2

Given an array of integers, return a new array such that each element at index i of the new array is the product of all the numbers in the original array except the one at i.
For example, if our input was [1, 2, 3, 4, 5], the expected output would be [120, 60, 40, 30, 24]. If our input was [3, 2, 1], the expected output would be [2, 3, 6].
Follow-up: what if you can't use division?

Solution #2

Problem 3

Given the root to a binary tree, implement serialize(root), which serializes the tree into a string, and deserialize(s), which deserializes the string back into the tree.
For example, given the following Node class.

class Node:
	def __init__(self, val, left=None, right=None):
		self.val = val
		self.left = left
		self.right = right

The following test should pass:

node = Node('root', Node('left', Node('left.left')), Node('right'))
assert deserialize(serialize(node)).left.left.val == 'left.left'

Solution #3 (only serialisation)

Problem 4

Given an array of integers, find the first missing positive integer in linear time and constant space. In other words, find the lowest positive integer that does not exist in the array. The array can contain duplicates and negative numbers as well.
For example, the input [3, 4, -1, 1] should give 2. The input [1, 2, 0] should give 3.
You can modify the input array in-place.

Solution #4

Problem 5

cons(a, b) constructs a pair, and car(pair) and cdr(pair) returns the first and last element of that pair. For example, car(cons(3, 4)) returns 3, and cdr(cons(3, 4)) returns 4.
Given this implementation of cons:

def cons(a, b):
	def pair(f):
		return f(a, b)
	return pair

Implement car and cdr.

Solution #5

Problem 6

An XOR linked list is a more memory efficient doubly linked list. Instead of each node holding next and prev fields, it holds a field named both, which is an XOR of the next node and the previous node. Implement an XOR linked list; it has an add(element) which adds the element to the end, and a get(index) which returns the node at index.
If using a language that has no pointers (such as Python), you can assume you have access to get_pointer and dereference_pointer functions that converts between nodes and memory addresses.

Solution #6

Problem 7

Given the mapping a = 1, b = 2, ... z = 26, and an encoded message, count the number of ways it can be decoded.
For example, the message 111 would give 3, since it could be decoded as aaa, ka, and ak.
You can assume that the messages are decodable. For example, 001 is not allowed.

Solution #7

Problem 8

A unival tree (which stands for "universal value") is a tree where all nodes under it have the same value.
Given the root to a binary tree, count the number of unival subtrees.
For example, the following tree has 5 unival subtrees:

   0
  / \
 1   0
    / \
   1   0
  / \
 1   1

Solution #8

Problem 10

Implement a job scheduler which takes in a function f and an integer n, and calls f after n milliseconds.

Solution #10

Problem 14

The area of a circle is defined as πr^2. Estimate π to 3 decimal places using a Monte Carlo method.
Hint: The basic equation of a circle is x^2 + y^2 = r^2.

Solution #14

Problem 16

You run an e-commerce website and want to record the last N order ids in a log. Implement a data structure to accomplish this, with the following API:

• record(order_id): adds the order_id to the log
• get_last(i): gets the ith last element from the log. i is guaranteed to be smaller than or equal to N. You should be as efficient with time and space as possible.

Solution #16