2657. Find the Prefix Common Array of Two Arrays

Difficulty: Medium

Topics: Array, Hash Table, Bit Manipulation

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.

Hint:

1. Consider keeping a frequency array that stores the count of occurrences of each number till index i.
2. If a number occurred two times, it means it occurred in both A and B since they’re both permutations so add one to the answer.

Solution:

We can iterate over the two arrays A and B while keeping track of the numbers that have occurred at or before the current index in both arrays. Since both arrays are permutations of the same set of numbers, we can utilize two hash sets (or arrays) to store which numbers have appeared at or before the current index in both arrays. For each index, we can count the common numbers that have appeared in both arrays up to that point.

Solution Approach:

1. Use two arrays to keep track of the occurrences of numbers in both A and B up to index i.
2. For each index i, check if both A[i] and B[i] have been seen before. If so, increment the common count.
3. Use a frequency array to track the presence of numbers from 1 to n in both arrays.

Let's implement this solution in PHP: 2657. Find the Prefix Common Array of Two Arrays

<?php
/**
 * @param Integer[] $A
 * @param Integer[] $B
 * @return Integer[]
 */
function findThePrefixCommonArray($A, $B) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Example usage:
$A = [1, 3, 2, 4];
$B = [3, 1, 2, 4];
print_r(findThePrefixCommonArray($A, $B)); // Output: [0, 2, 3, 4]

$A = [2, 3, 1];
$B = [3, 1, 2];
print_r(findThePrefixCommonArray($A, $B)); // Output: [0, 1, 3]
?>




<h3>
  
  
  Explanation:
</h3>

<ol>
<li>
<strong>Frequency Arrays</strong>: We maintain two frequency arrays, freqA and freqB, where each index represents a number in the permutation.

<ul>
<li>When we encounter a number in A[i] or B[i], we increase the corresponding value in the frequency array.</li>
</ul>
</li>
<li>
<strong>Common Count</strong>: After updating the frequency arrays for both A[i] and B[i], we check for each number if it has appeared in both arrays up to index i. If so, we increase the commonCount.</li>
<li>
<strong>Result</strong>: The common count is stored in the result array for each index.</li>
</ol>

<h3>
  
  
  Example Walkthrough:
</h3>

<p>For the input:<br>
</p>

<pre class="brush:php;toolbar:false">$A = [1, 3, 2, 4];
$B = [3, 1, 2, 4];

• At i = 0: No common numbers yet → C[0] = 0
• At i = 1: Numbers 1 and 3 are common → C[1] = 2
• At i = 2: Numbers 1, 2, and 3 are common → C[2] = 3
• At i = 3: Numbers 1, 2, 3, and 4 are common → C[3] = 4

Output: [0, 2, 3, 4]

Time Complexity:

• O(n²⁾: For each index i, we are checking every element from 1 to n to see if it's common, making this solution quadratic in time complexity. This is acceptable given the constraint n ≤ 50.

This should work within the given constraints effectively.

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