Valid Anagram

The problem

Given two strings s and t, return true if t is an anagram of s, and false otherwise.

Two strings are anagrams if and only if they have identical length and identical character counts. A single fixed-size array acts as a balance scale: increment counts for letters in s and decrement for letters in t. If any count ends nonzero or goes negative early, the strings cannot be anagrams.

Full solution

Fixed 26-slot frequency balance array: increment for s, decrement for t, bail the moment a count goes negative. Same time as sorting but O(1) space and a single pass each way.

def is_anagram(s: str, t: str) -> bool:
    if len(s) != len(t):
        return False
    counts = [0] * 26
    a = ord("a")
    for ch in s:
        counts[ord(ch) - a] += 1
    for ch in t:
        idx = ord(ch) - a
        counts[idx] -= 1
        if counts[idx] < 0:
            return False
    return True

function isAnagram(s, t) {
  if (s.length !== t.length) return false;
  const counts = new Array(26).fill(0);
  const aCode = "a".charCodeAt(0);
  for (const ch of s) counts[ch.charCodeAt(0) - aCode]++;
  for (const ch of t) {
    const idx = ch.charCodeAt(0) - aCode;
    counts[idx]--;
    if (counts[idx] < 0) return false;
  }
  return true;
}

The theory behind it

Strings — the ground this problem stands on. All Strings problems

What Strings is

A string is an ordered necklace of text characters, like letters printed along a ribbon of paper. Each character sits at an exact numeric slot, holding a glyph such as a letter, punctuation mark, or digit. In many programming languages, ribbons cannot be edited after creation, meaning changing a single character requires pressing an entirely new ribbon from scratch.

When to reach for it

Reach for string techniques when inputs consist of words, DNA sequences, serialized data formats, or sentences. Clues include questions testing palindromes, anagram matches, substring patterns, parenthesis balancing, or character frequency counts. Whenever an algorithm asks to transform capitalization, parse structured tokens, or compute edits between two phrases, string representations are the core subject.

How the pattern works

Think of characters as small integer codes ranging across standard character sets. Frequency tables with fixed sizes often replace heavy hash maps when tallying occurrences. For search tasks, maintain rolling state using character indices or sliding borders. When building output text through repeated appends, accumulate pieces inside a mutable list or string builder rather than concatenating strings directly, avoiding quadratic copy overhead.

What each operation costs

Which roles need this problem

Strings is a core topic for these 13 roles — if you're targeting one of them, this problem is early in your path, not optional.

Secondary for 7 more roles, including Data Engineer, Data Analyst, Embedded / Firmware Engineer.