Leetcode problem 68 - Find the Prefix Common Array of Two Arrays

You are given two 0-indexed integer permutations A and B of length n.

A prefix common array of A and B is an array C such that C[i] is equal to the count of numbers that are present at or before the index i in both A and B.

Return the prefix common array of A and B.

A sequence of n integers is called a permutation if it contains all integers from 1 to n exactly once.

Example 1:

Input: A = [1,3,2,4], B = [3,1,2,4] Output: [0,2,3,4] Explanation: At i = 0: no number is common, so C[0] = 0. At i = 1: 1 and 3 are common in A and B, so C[1] = 2. At i = 2: 1, 2, and 3 are common in A and B, so C[2] = 3. At i = 3: 1, 2, 3, and 4 are common in A and B, so C[3] = 4. Example 2:

Input: A = [2,3,1], B = [3,1,2] Output: [0,1,3] Explanation: At i = 0: no number is common, so C[0] = 0. At i = 1: only 3 is common in A and B, so C[1] = 1. At i = 2: 1, 2, and 3 are common in A and B, so C[2] = 3.

/**
 * @param {number[]} A
 * @param {number[]} B
 * @return {number[]}
 */
var findThePrefixCommonArray = function (A, B) {
  // create a hash table
  let hash = {};
  // create a result array
  let result = [];
  // create a count variable
  let count = 0;

  // loop through the array
  for (let i = 0; i < A.length; i++) {
    // if the element is in the hash table
    if (A[i] in hash) {
      // increment the count
      count++;
    } else {
      // add the element to the hash table
      hash[A[i]] = 0;
    }
    // push the count to the result array
    if (B[i] in hash) {
      // increment the count
      count++;
    } else {
      // add the element to the hash table
      hash[B[i]] = 0;
    }
    //  push the count to the result array
    result.push(count);
  }
  // return the result array
  return result;
};

The time complexity of this function is O(n), where n is the length of the input arrays A and B, because it performs a single pass through both arrays, processing each element once.