Intersection of Two arrays with Duplicate Elements

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Problem Overview

Find intersection counting multiplicities.

See our study guide for structured GFG and LeetCode practice.

Intuition

Find intersection counting multiplicities. Sort both, two-pointer or frequency maps.

Algorithm

1Sort both. Two pointers: advance smaller, collect equal elements.

Common Pitfalls

• Same as LC 350. Each element in result appears as many times as it appears in both arrays.

Intersection of Two arrays with Duplicate Elements.java

Java

// Approach: Sort both arrays or use frequency maps; collect common elements.
// Time: O(n log n + m log m) Space: O(min(n,m))
class Solution {
    public ArrayList<Integer> intersectionWithDuplicates(int[] a, int[] b) {
        ArrayList<Integer> li = new ArrayList<>();
        int n = a.length;
        int m = b.length;
        HashSet<Integer> hs1 = new HashSet<>();
        HashSet<Integer> hs2 = new HashSet<>();
        for (int i = 0; i < n; i++) {
            int temp = a[i];
            hs1.add(temp);
        }

        for (int i = 0; i < m; i++) {
            int temp = b[i];
            hs2.add(temp);
        }

        for (int key : hs1) {
            if (hs2.contains(key))
                li.add(key);
        }

        return li;
    }
}

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