Patterns

Slow read pointer + fast write pointer (or different speeds), both moving forward

When to useIn-place array filtering/dedup, reading at one rate while writing at another, cycle detection

iterative

INITIALIZE

slow=0; fast=0 (or different rates)

let slow = 0; let fast = 0;

const vowels = 'aeiouAEIOU'; const out = [];

let result = ''; let slow = 0;

let slow = 0; let fast = 0;

let slow = 0; let fast = 0;

let slow = 0; let fast = 0;

const res = s.split(''); let slow = 2;

—

TRAVERSE

for (fast = 0; fast < n; fast++)

for (let fast = 0; fast < nums.length; fast++) {

for (const char of s) {

for (let fast = 1; fast <= s.length; fast++) {

for (let fast = 0; fast < arr.length; fast++) {

for (let fast = 0; fast < arr.length; fast++) {

for (; fast < s.length; fast++) {

for (let fast = 2; fast < s.length; fast++) {

—

[DECIDE]

decide whether to keep current fast value

  if (nums[fast] !== target) {

  if (!vowels.includes(char)) {

  if (fast === s.length || s[fast] !== s[slow]) {

  if (arr[fast] !== 0) {

  if (arr[fast] !== arr[slow]) {

  // decide whether to keep s[fast] (run-length grouping, not canonical slow/fast)

  if (s[fast] !== res[slow - 1] || s[fast] !== res[slow - 2]) {

—

write+advance

if keep, write to slow position; advance slow

    nums[slow] = nums[fast]; slow++;

    out.push(char);

    result += s[slow] + (fast - slow); slow = fast;

    arr[slow] = arr[fast]; slow++;

    slow++; arr[slow] = arr[fast];

  // append kept chars; advance slow

    res[slow] = s[fast]; slow++;

—

RETURN

return slow (often new length or transformed value)

return nums.slice(0, slow);

return out.join('');

return result;

// fill tail with zeros, then return arr

return slow + 1;

return ans;

return res.slice(0, slow).join('');

—