# Find the Prefix Common Array of Two Arrays

# 2657. 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. ```

Constraints: ``` 1 <= A.length == B.length == n <= 50 1 <= A[i], B[i] <= n It is guaranteed that A and B are both a permutation of n integers. ```

/** * @param {number[]} A * @param {number[]} B * @return {number[]} */ const findThePrefixCommonArray = function(A, B) { const aSet = new Set(), bSet = new Set(), usedSet = new Set(), out = []; let count = 0; const markAsUsed = (element, setObj) => { if (setObj.has(element) && !usedSet.has(element)) { count++; usedSet.add(element); } } A.forEach((elementA, index) => { markAsUsed(A[index], bSet.add(B[index])); markAsUsed(B[index], aSet.add(A[index])); out.push(count); }) return out; };