Two pointers

Two pointers: one input, opposite ends

JavaScript

let fn = arr => {
    let left = 0, ans = 0, right = arr.length - 1;

    while (left < right) {
        // do some logic here with left and right
        if (/* CONDITION */) {
            left++;
        } else {
            right--;
        }
    }

    return ans;
}

TypeScript

let fn = (arr: any[]): number => {
    let left: number = 0, ans: number = 0, right: number = arr.length - 1;

    while (left < right) {
        // do some logic here with left and right
        if (/* CONDITION */) {
            left++;
        } else {
            right--;
        }
    }

    return ans;
}

Two pointers: two inputs, exhaust both

JavaScript

let fn = (arr1, arr2) => {
    let i = 0, j = 0, ans = 0;

    while (i < arr1.length && j < arr2.length) {
        // do some logic here
        if (/* CONDITION */) {
            i++;
        } else {
            j++;
        }
    }

    while (i < arr1.length) {
        // do logic
        i++;
    }

    while (j < arr2.length) {
        // do logic
        j++;
    }

    return ans;
}

TypeScript

let fn = (arr1: any[], arr2: any[]): number => {
    let i: number = 0, j: number = 0, ans: number = 0;

    while (i < arr1.length && j < arr2.length) {
        // do some logic here
        if (/* CONDITION */) {
            i++;
        } else {
            j++;
        }
    }

    while (i < arr1.length) {
        // do logic
        i++;
    }

    while (j < arr2.length) {
        // do logic
        j++;
    }

    return ans;
}

Examples

Second pointer start when string ends

var reverseWords = function (s) {
  const reversedWords = [];
  let index = s.indexOf(" ");
  let first = index - 1;
  let second = 0;
  while (second < s.length) {
    if (first === -2) {
      first = s.length - 1;
    }
    if (second !== index) {
      reversedWords[second] = s[first];
      first--;
    } else {
      reversedWords[second] = " ";
      index = s.indexOf(" ", index + 1);
      first = index - 1;
    }
    second++;
  }
  return reversedWords.join("");
};

Get common val in two arrays

var getCommon = function (nums1, nums2) {
  let first = 0;
  let second = 0;

  while (first < nums1.length && second < nums2.length) {
    if (nums1[first] === nums2[second]) {
      return nums1[first];
    }

    if (nums1[first] > nums2[second]) {
      second++;
    } else if (nums2[second] > nums1[first]) {
      first++;
    }
  }

  return -1;
};