Character Counting

Most Frequent Character — Hash Map

Easy • Arrays & Hashing

Given a string s, count the frequency of each character and return the most frequent character. If there's a tie, return the one that appears first.

TimeO(n)two passes

SpaceO(k)hash map, k = unique chars

Phase: — Index: — Result: —

Current index

In hash map

Best so far

map —

result —

maxCount —

Speed:

Ready

Press Play to watch. Pass 1: count each character in a hash map. Pass 2: scan left-to-right, return first char with max count.

✎ Whiteboard

Size: 3

⌨ Type It — Build Muscle Memory

Practice until you don't need to look. Use the guide comments below as scaffolding. The green highlights are the nuances to burn into memory.

# ─── MOST FREQUENT CHARACTER ───

# Pattern: Two passes — count, then find max (first wins on tie)

# Time: O(n) Space: O(k)

# 1. Define: most_frequent(s)

# 2. Pass 1 — count chars: map = {}

# for c in s: map[c] = map.get(c, 0) + 1

# 3. Pass 2 — find max (left-to-right so first wins on tie):

# result = s[0]; maxCount = 0

# for c in s:

# if map[c] > maxCount: result, maxCount = c, map[c]

# 4. Return: return result

# Vars: map (dict), result, maxCount, c

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