Count the Occurrences of Each Character

📌 Definition:

📖 Detailed Explanation:

Scan once, accumulating counts in a hash map keyed by character. A single pass is O(n). For fixed ASCII input an int[256] frequency array avoids hashing overhead. Use an ordered map if first-seen order must be preserved.

💻 Solutions:

• Java
• JavaScript
• Python

public static Map<Character,Integer> countChars(String s) {
    Map<Character,Integer> m = new LinkedHashMap<>();
    for (char c : s.toCharArray()) m.merge(c, 1, Integer::sum);
    return m;
}

function countChars(s) {
  const m = {};
  for (const c of s) m[c] = (m[c] || 0) + 1;
  return m;
}

from collections import Counter

def count_chars(s):
    return dict(Counter(s))

🔑 Key Points:

• Single pass with a hash map — O(n)
• int[256]/int[128] array for ASCII
• Ordered map preserves first-seen order
• Foundation for anagram / unique-char problems

🌍 Real-World Example: