🚀 90 Days of DSA – Day 7: Strings & Arrays Deep Dive

📅 Day 7 of my #90DaysOfDSA journey was packed with string and array-based problems. From detecting duplicates to checking isomorphism between strings — it was all about pattern recognition and data structure efficiency.

🔗 Full Repo Link: DSA-Problems → Day 7

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✅ Problem 1: Contains Duplicate

🔗 View Code

pythonCopyEditclass Solution:
    def containsDuplicate(self, nums: List[int]) -> bool:
        return len(set(nums)) < len(nums)

📌 Logic: Convert the list to a set and compare lengths. If set size is smaller, duplicates exist.

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✅ Problem 2: First Unique Character in a String

🔗 View Code

from collections import Counter
class Solution:
    def firstUniqChar(self, s: str) -> int:
        dict1=Counter(s)
        for i in dict1.keys():
            if dict1[i] == 1:
                return s.index(i)
        return -1

📌 Logic: Iterate through characters and return the index of the first character that appears exactly once.

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✅ Problem 3: Isomorphic Strings

🔗 View Code

pythonCopyEditclass Solution:
    def isIsomorphic(self, s: str, t: str) -> bool:
        return len(set(s)) == len(set(t)) == len(set(zip(s, t)))

📌 Logic: Ensure the number of unique characters and their mappings match using set(zip(s, t)).

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✅ Problem 4: Valid Anagram

🔗 View Code

class Solution:
    def isAnagram(self, s: str, t: str) -> bool:
        if len(s)!=len(t):
            return False
        sets=set(s)
        for i in sets:
            if s.count(i)!=t.count(i):
                return False
        return True

📌 Logic: Check if both strings have the same character counts.

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✅ Problem 5: Intersection of Two Arrays II

🔗 View Code

from collections import Counter

class Solution:
    def intersect(self, nums1: List[int], nums2: List[int]) -> List[int]:
        count1 = Counter(nums1)
        result = []

        for num in nums2:
            if count1[num] > 0:
                result.append(num)
                count1[num] -= 1

        return result

📌 Logic: Use Counter to track occurrences and build the intersection using minimum counts.

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🔗 All Code Files

Browse and fork the complete Day 7 progress here:
📁 GitHub Repo – Day 7

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📈 Reflections

• Practiced frequency-based solutions and mapping logic

• Learned more about efficient Python tricks like set(zip(s, t))

• Starting to spot patterns quickly in string and array problems!

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🚀 Connect with Me

Follow my journey on Twitter @CodeWithPiyush and explore my projects and tools built with AI and Python.