valid-anagram.ts

/**
 * 242. Valid Anagram  (Easy)
 * Link: https://leetcode.com/problems/valid-anagram/
 *
 * Given two strings `s` and `t`, return true if `t` is an anagram of `s`
 * (uses exactly the same characters with the same frequencies).
 *
 * Example:
 *   Input:  s = "anagram", t = "nagaram"
 *   Output: true
 *
 * Approach:
 *   Anagrams must have identical character counts. Tally `s` in a map, then
 *   decrement while scanning `t`; if any count goes negative or a character is
 *   missing, or lengths differ, they are not anagrams.
 *
 * Time:  O(n)  — two passes over the strings.
 * Space: O(k)  — k = size of the character alphabet.
 */
export function isAnagram(s: string, t: string): boolean {
  if (s.length !== t.length) return false;

  const counts = new Map<string, number>();
  for (const ch of s) {
    counts.set(ch, (counts.get(ch) ?? 0) + 1);
  }

  for (const ch of t) {
    const remaining = counts.get(ch);
    if (remaining === undefined || remaining === 0) return false;
    counts.set(ch, remaining - 1);
  }

  return true;
}