Top 10 String Problems for FAANG Interviews

Here’s your Top 10 String Problems for FAANG Interviews, with focus on patterns, reasoning, and what interviewer is testing (plus optional quick Python solution hints).

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🧠 Top 10 String Problems for FAANG Interviews

#	Problem	Core Pattern / Concept	Key Skills Tested	Difficulty
1️⃣	Longest Substring Without Repeating Characters	Sliding Window + HashMap	Handling window shrink/expand efficiently	Medium
2️⃣	Valid Anagram	Hash Map / Sorting	Frequency counting	Easy
3️⃣	Group Anagrams	Hash Map of sorted keys	Hashing logic with string manipulation	Medium
4️⃣	Longest Palindromic Substring	Expand Around Center / DP	Palindrome logic, dynamic programming	Medium
5️⃣	Palindrome Check / Valid Palindrome II	Two Pointers	Ignoring non-alphanumeric chars, skipping one	Easy–Medium
6️⃣	String Compression / Run-Length Encoding	Two Pointers / Counting	In-place string manipulation	Medium
7️⃣	Minimum Window Substring	Sliding Window + HashMap	Hardest window problem (two-pointer mastery)	Hard
8️⃣	Roman to Integer / Integer to Roman	Mapping + Iteration	Lookup logic	Easy–Medium
9️⃣	Longest Common Prefix	Sorting + Comparison	String traversal logic	Easy
🔟	Encode and Decode Strings (LeetCode 271)	Custom encoding/decoding logic	Serialization reasoning	Medium–Hard

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⚙️ Quick Python Strategy & Snippets

Problem	Quick Idea (1-liner Strategy)	Python Hint
1. Longest Substring Without Repeating	Maintain window and move left when duplicate seen	s, seen, l, ans = "abcabcbb", {}, 0, 0\nfor r, c in enumerate(s):\n if c in seen and seen[c]>=l: l = seen[c]+1\n seen[c]=r; ans=max(ans, r-l+1)
2. Valid Anagram	Count chars via Counter	from collections import Counter\nreturn Counter(s)==Counter(t)
3. Group Anagrams	Sort each word as key	from collections import defaultdict\nmp=defaultdict(list)\nfor w in strs: mp[''.join(sorted(w))].append(w)\nreturn mp.values()
4. Longest Palindromic Substring	Expand from each center	def expand(l,r):\n while l>=0 and r<len(s) and s[l]==s[r]: l-=1; r+=1; return s[l+1:r]\nres=max((expand(i,i),expand(i,i+1)) for i in range(len(s)), key=len)
5. Valid Palindrome II	Two pointers; at most one mismatch	def validPalindrome(s):\n l,r=0,len(s)-1\n while l<r:\n if s[l]!=s[r]: return s[l+1:r+1]==s[l+1:r+1][::-1] or s[l:r]==s[l:r][::-1]\n l+=1;r-=1\n return True
6. String Compression	Count consecutive chars	s+=" "; res=""; c=1\nfor i in range(1,len(s)):\n if s[i]==s[i-1]: c+=1\n else: res+=s[i-1]+(str(c) if c>1 else ""); c=1
7. Minimum Window Substring	Slide window until all chars matched	need=Counter(t);missing=len(t);l=start=end=0\nfor r,c in enumerate(s,1): missing-=need[c]>0;need[c]-=1\n while not missing: need[s[l]]+=1; missing+=need[s[l]]>0; l+=1\n if not end or r-l<end-start:start,end=l-1,r
8. Roman to Integer	Add/subtract based on next symbol	val={'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000}\nres=0\nfor i in range(len(s)):\n res+=val[s[i]]*(-1 if i+1<len(s) and val[s[i]]<val[s[i+1]] else 1)
9. Longest Common Prefix	Compare first and last after sorting	strs.sort();s1,s2=strs[0],strs[-1]\ni=0\nwhile i<len(s1) and s1[i]==s2[i]: i+=1\nreturn s1[:i]
10. Encode/Decode Strings	Prefix with length of each word	def encode(strs): return ''.join(f"{len(s)}#{s}" for s in strs)\ndef decode(s): res,i=[],0\nwhile i<len(s): j=s.find('#',i); l=int(s[i:j]); res.append(s[j+1:j+1+l]); i=j+1+l

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🧩 String Problem Patterns to Master

Pattern	Example Problems
Sliding Window	Longest Substring, Minimum Window
Two Pointers	Palindrome, String Compression
Hash Map / Frequency	Anagram, Group Anagrams
Expand Around Center	Longest Palindrome
Sorting / Comparison	Common Prefix, Group Anagrams

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🧠 Bonus “Must Know” String Topics

• ASCII & Unicode handling
• Mutable vs immutable strings
• StringBuilder in Java (for performance)
• Common built-in methods: split(), join(), startswith(), endswith(), find()