DSA Sheet - Day 6 Strings

🚀 DSA Sheet – Day 6: Strings (Part 1)

Welcome to Strings! This section focuses on pattern recognition, frequency counting, and sliding window techniques — all extremely common in interviews.

📊 Progress Tracker

Progress: 0 / 6 Completed

Done	Problem	Practice	Level	Companies
	Valid Palindrome	Solve	Easy	Cisco, Zoho, Paytm
	Valid Anagram	Solve	Easy	Google, Directi
	Reverse Words in String	Solve	Medium	Amazon, Adobe
	Remove All Occurrences	Solve	Medium	Google, Microsoft
	Permutation in String	Solve	Medium	Uber, Goldman Sachs
	String Compression	Solve	Medium	Amazon

🧠 Key Approaches

• Valid Palindrome: Two pointer, ignore non-alphanumeric
• Valid Anagram: Frequency count (array/hashmap)
• Reverse Words: Trim + reverse logic
• Remove Occurrences: Stack or string simulation
• Permutation in String: Sliding window + frequency match
• String Compression: Two pointer (in-place write)

⚡ Quick Cheatsheet

Valid Palindrome

int l = 0, r = s.size() - 1;

while(l < r){
  if(!isalnum(s[l])) l++;
  else if(!isalnum(s[r])) r--;
  else if(tolower(s[l++]) != tolower(s[r--])) 
    return false;
}
    

Valid Anagram

int count[26] = {0};

for(char c : s) count[c - 'a']++;
for(char c : t) count[c - 'a']--;
    

Permutation in String (Sliding Window)

vector s1(26), s2(26);

for(char c : s1) s1[c - 'a']++;

for(int i = 0; i < s2.size(); i++){
  s2[s2[i] - 'a']++;
}
    

🔥 Pro Tip

Strings are all about patterns:

• Two Pointer Traversal
• Sliding Window
• Frequency Counting

Once you recognize patterns, most string problems become predictable.

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