const word = "learn";

console.log(word[0]); // "l"
console.log(word[4]); // "n"
console.log(word[9]); // undefined

const text = "abc";

for (let i = 0; i < text.length; i += 1) {
  console.log(i, text[i]);
}

for (const ch of text) {
  console.log(ch);
}

const username = "arraya";
const len = username.length; // 6

const a = "Hello";
const b = "hello";

console.log(a === b); // false
console.log(a.toLowerCase() === b.toLowerCase()); // true

const first = "Data";
const second = "Structure";

const titleA = first + " " + second;
const titleB = `${first} ${second}`;

const text = "algorithm";

console.log(text.slice(0, 5)); // "algor"
console.log(text.slice(5)); // "ithm"

const text = "learn data structures";

text.includes("data"); // true
text.indexOf("a"); // 2
text.indexOf("z"); // -1

const text = "I like JS";
const updated = text.replace("JS", "TypeScript");
// "I like TypeScript"

const text = "Hello";
text.toLowerCase(); // "hello"
text.toUpperCase(); // "HELLO"

const raw = "   hello world   ";
const clean = raw.trim(); // "hello world"

const csv = "red,green,blue";
const colors = csv.split(","); // ["red", "green", "blue"]

const words = ["learn", "string", "today"];
const sentence = words.join(" "); // "learn string today"

const text = "cat";
// text[0] = "b"; // ไม่สามารถแก้แบบนี้ได้
const updated = "b" + text.slice(1); // "bat"

function countChars(text: string) {
  const freq = new Map<string, number>();
  for (const ch of text) {
    freq.set(ch, (freq.get(ch) ?? 0) + 1);
  }
  return freq;
}

function isPalindrome(text: string) {
  const s = text.toLowerCase();
  let left = 0;
  let right = s.length - 1;

  while (left < right) {
    if (s[left] !== s[right]) return false;
    left += 1;
    right -= 1;
  }
  return true;
}

function reverseString(text: string) {
  return text.split("").reverse().join("");
}

function isAnagram(a: string, b: string) {
  if (a.length !== b.length) return false;

  const freq = new Map<string, number>();
  for (const ch of a) freq.set(ch, (freq.get(ch) ?? 0) + 1);
  for (const ch of b) {
    const left = (freq.get(ch) ?? 0) - 1;
    if (left < 0) return false;
    freq.set(ch, left);
  }
  return true;
}

const text = "datastructure";
const subA = text.substring(0, 4); // "data"
const subB = text.slice(4, 13); // "structure"

const filename = "report.pdf";

filename.startsWith("rep"); // prefix
filename.endsWith(".pdf"); // suffix

const text = "order-2026-04";
const hasOrderPattern = /^order-\d{4}-\d{2}$/.test(text); // true

function longestWithoutRepeat(text: string) {
  let left = 0;
  let best = 0;
  const last = new Map<string, number>();

  for (let right = 0; right < text.length; right += 1) {
    const ch = text[right];
    if (last.has(ch) && (last.get(ch) ?? -1) >= left) {
      left = (last.get(ch) ?? left) + 1;
    }
    last.set(ch, right);
    best = Math.max(best, right - left + 1);
  }
  return best;
}