Frequency Counting
DSA · Hash Maps & Setssyntax
Build a map of element → count.
Useful for: most common element, anagrams, character counting, voting problems.example
// JavaScript
function frequencyMap(arr) {
const freq = new Map();
for (const item of arr) {
freq.set(item, (freq.get(item) || 0) + 1);
}
return freq;
}
function mostFrequent(arr) {
const freq = frequencyMap(arr);
let maxCount = 0, result = null;
for (const [key, count] of freq) {
if (count > maxCount) { maxCount = count; result = key; }
}
return result;
}
# Python
from collections import Counter
def frequency_map(arr):
freq = {}
for item in arr:
freq[item] = freq.get(item, 0) + 1
return freq
def most_frequent(arr):
return Counter(arr).most_common(1)[0][0]output
most_frequent([1,2,2,3,3,3]) → 3Note Time O(n), Space O(k) where k is unique elements. Python's Counter is a powerful shortcut. In interviews, building the map manually shows understanding. Tie-breaking: clarify with interviewer what to return if multiple elements share max frequency.