#️⃣ Hashing (Hashmaps & Sets)#

Hash maps offer O(1) average insert, lookup, and delete. Sets track existence; maps track frequency or mapping.

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Core Patterns#

Pattern	Tool	Example
Existence check	unordered_set	Duplicates, anagram detection
Frequency count	unordered_map<T,int>	Count chars, top-k elements
Two-sum lookup	unordered_map<T,int>	Find pair that sums to target
Grouping	unordered_map<key, vector>	Group anagrams by sorted key
Prefix sum + map	unordered_map<int,int>	Count subarrays with target sum

Templates#

// Frequency count
unordered_map<int, int> freq;
for (int x : arr) freq[x]++;

// Two-sum lookup
unordered_map<int, int> seen; // value → index
for (int i = 0; i < nums.size(); i++) {
    if (seen.count(target - nums[i]))
        return {seen[target - nums[i]], i};
    seen[nums[i]] = i;
}

// Group anagrams
unordered_map<string, vector<string>> groups;
for (string& s : strs) {
    string key = s; sort(key.begin(), key.end());
    groups[key].push_back(s);
}

Complexity#

Operation	Average	Worst
Insert / Lookup / Delete	O(1)	O(n)
Space	O(n)	O(n)

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