In computer science, a string is a sequence of characters. While they behave similarly to arrays, they have unique properties (like immutability in some languages) that require specific algorithmic thinking.

Think of a string as a beaded necklace:

1. Each bead is a character ('a', '!', ' ').
2. You can read the order of beads easily.
3. In many languages, if you want to change one bead, you actually have to make a whole new necklace with the change. This is called Immutability.

---

Immutability & Performance

In languages like JavaScript, Java, and Python, strings are immutable.

• Doing str += "new char" in a loop actually creates $n$ temporary strings!
• Complexity: $O(n^2)$ if not careful.
• Solution: Use an array and join('') at the end, or a StringBuilder.

Common String Operations

Important ASCII Knowledge

Characters are just numbers. Knowing the ranges can help you manipulate them:

• 'a' to 'z': 97 - 122
• 'A' to 'Z': 65 - 90
• '0' to '9': 48 - 57

---

javascript Standard
/**
 * REVERSING A STRING
 * Since strings are immutable, we convert to array first.
 */
function reverseString(str) {
  // Convert to array, reverse, then join
  return str.split('').reverse().join('');
}

/**
 * CHECK IF TWO STRINGS ARE ANAGRAMS
 * Anagram: Words with the same characters in different order (e.g., 'listen', 'silent')
 */
function isAnagram(s1, s2) {
  if (s1.length !== s2.length) return false;

  // Use a frequency map (O(n) time, O(1) space for 26 chars)
  const count = new Array(26).fill(0);
  
  for (let i = 0; i < s1.length; i++) {
    count[s1.charCodeAt(i) - 97]++; // Increment for s1
    count[s2.charCodeAt(i) - 97]--; // Decrement for s2
  }

  // If all are zero, it's an anagram
  return count.every(val => val === 0);
}

---

Key Patterns

• Frequency Map: Always the first thought for character counting.
• Two Pointers: Used for reversal, palindromes, and removing whitespace.
• In-Place Modification: If the input is a character array char[], try to do it with O(1) space.

Common Questions

• Reverse words in a sentence (e.g., "Hello World" -> "World Hello").
• Longest Palindromic Substring.
• Group Anagrams (using a HashMap).
• Valid Parentheses. Blackwell