#4 Medium Time O(m·n·log n) Space O(m·n) Animated

Group Anagrams

Group an array of strings so that anagrams are together.

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1 Predict
2 Trace
3 Explain
4 Implement

Pattern Hashing for constant-time memory

Trade one linear scan for a small lookup structure so each value is handled once.

Approach: Hash Map with Character Count Array

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Core Insight

Trade one linear scan for a small lookup structure so each value is handled once.

Common Pitfall

Do not overwrite information you still need before checking the current value.

Description

Group an array of strings so that anagrams are together.

Constraints

Time Complexity: O(m·n·log n)
Space Complexity: O(m·n)

Solution — Hash Map with Character Count Array

TimeO(m·n·log n) SpaceO(m·n)

Implementation Plan

Translate the invariant into these small, testable moves.

Create the lookup structure.
Scan each value once.
Check the needed relationship before updating state.
Return as soon as the answer is determined.

def groupAnagrams(strs):
    res = defaultdict(list)
    for s in strs:
        count = [0] * 26
        for c in s:
            count[ord(c) - ord('a')] += 1
        res[tuple(count)].append(s)
    return list(res.values())

Two Sum

Top K Frequent Elements

Group Anagrams

Steps

Description

Constraints

Tags

Solution — Hash Map with Character Count Array