Hashing & FrequencyBeginner
Hash Map Counting
Use a hash map to count frequencies or track seen elements for O(1) lookups.
Problem: First Unique Character in a String
Find the first non-repeating character in "leetcode" and return its index.
l
e
e
t
c
o
d
e
i
Frequency Map
'l': 1
Count char 'l' at index 0
Start building frequency map. First character 'l' appears once so far.
Step 1/10
Speed:
When to Use
- Count element frequencies
- Check if element exists
- Find duplicates
- Group anagrams
Key Indicators
- Frequency counting
- "Find duplicates"
- Group by property
- O(1) lookup needed
Complexity
Time
O(n)
Space
O(n)
Common Problems
First Unique Character in a StringGroup AnagramsTop K Frequent ElementsValid AnagramContains Duplicate
Code
freq = {}
for element in arr:
freq[element] = freq.get(element, 0) + 1
# Now freq[x] gives count of x in O(1)Want all 29 patterns with detailed visual walkthroughs?
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