LeetCode #242: Valid Anagrams (Solution in Python & Explanation)

The Problem

Given two strings s and t, return true if t is an anagram of s, and false otherwise.

An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.

Example 1:

Input: s = "anagram", t = "nagaram"
Output: true

Example 2:

Input: s = "rat", t = "car"
Output: false

Solution

We know that the two strings are anagrams if:

  1. They have the same length
  2. They have the same characters and the same quantity of each character

So, if we can verify that the strings pass these two criteria, we can verify that they are valid anagrams.

We can check if they are the same length by simply using len:

len(s) == len(t)   # boolean expression

But how can we check if they have the same characters and the same quantity of each character? We can use hash maps. For each string, we can create a hash map that includes every unique character in the string and its frequency in the string.

For example, this would be the hash map of the string s, which has a value of "anagram":

s = "anagram"
countS = {"a": 3, "n": 1, "g": 1, "m": 1}

Once, we have created the hash maps, we can check if they are equivalent by using a boolean expression:

countS == countT

So, let’s put it all together. These are all the steps we have to take:

  1. We check if the strings are not the same length. If they’re not, we can immediately return False without doing any other code.
  2. We create empty hash maps for the two strings s and t.
  3. We loop through each character in s and t and add them to the hash maps. If the character does not exist in the hash map, a new key for the character is added and the value is initialized to 0 (then we add 1). If a key for the character does exist in the hash map, we add 1 to the value.
  4. We check if the two hash maps are equivalent. If they are, we return True. If they are not, we return False.

This is the final code.

   def isAnagram(self, s: str, t: str) -> bool:
        # Step 1: check if same length
        if len(s) != len(t):
            return False
        
        # Step 2: create hash maps
        countS = {}
        countT = {}
        
        # Step 3: loop through characters to add to hash map
        for i in range(len(s)):
            countS[s[i]] = 1 + countS.get(s[i], 0)
            countT[t[i]] = 1 + countT.get(t[i], 0)
        
        # Step 4: check if hash maps are equivalent  
        if (countS == countT): 
            return True
        else: 
            return False

This solution has a time complexity of O(s + t) or simply O(n) because we have to iterate through each of the strings. The space complexity is also O(s + t) because we are building hash maps that are the sizes of arrays s and t.

There is another solution that is much more simple, but less efficient. We can sort both of the strings and see if the sorted versions are equivalent. This works because, if they are anagrams, then they will have the same sorted value.

def isAnagram(self, s: str, t: str) -> bool:
    return sorted(s) == sorted(t)

A good sorting algorithm can sort an array of characters in O(n * log(n)) time. The space complexity could be either O(1) or O(n)

LeetCode: Two Sum (Solution in Python & Explanation)

In this article, I will be explaining how to solve the Two Sum problem on LeetCode. This is an Easy-level question for Arrays.

The Problem

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

The Solution

The most straightforward way to solve this problem is to check every combination of two values in the array and see if they can sum up to the target value. To do this, we would iterate through the entire array and for each item we would iterate through the array a second time to check the sums of the current array item with every other item in the array and see if they are the target. The (worst-case) runtime for this solution method is not very efficient: O(n2).

There is a better solution that only requires us to loop through the array once. Basically, if we could loop through the array and, for each array item, check if the number that adds with that value to get the target exists in the array using an operation that doesn’t have O(n) time complexity, then we could make our program much faster. We can do this by using a hash map.

We create a hash map that will store array items as keys and their indexes in the array as values ({array_value: array_index})

We use the enumerate function to get the index of the current item as we loop through the array. For each array item, we compute the difference diff between the target value and the item’s value. Since target - n = diff, that means diff + n = target. If this difference diff is in the hash map, then that means that the value has already been seen in the array. Thus, we’ve found the . If the difference is not in the array, then we add the current array item and its index to the hash map then move to the next array item.

def twoSum(self, nums: List[int], target: int) -> List[int]:
        hm = {} 
        for i, n in enumerate(nums): 
            diff = target - n 
            if diff in hm:
                    return [hm[diff], i]
            hm[n] = i

This solution has a time complexity of O(n) because we only have to loop through the nums array once and the lookup and insert operations into the hash map have a runtime of O(1). Although, we do have to use extra memory to create the hash map. The memory complexity is O(n) because we could potentially add every value to the hash map.