LeetCode 1865: Finding Pairs With a Certain Sum Solution

Master LeetCode problem 1865 (Finding Pairs With a Certain Sum), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

1865. Finding Pairs With a Certain Sum

Medium

Array Hash Table Design

Problem Explanation

Explanation

To solve this problem efficiently, we can use a hashmap to store the frequency of sums that can be obtained by adding elements from nums1 and nums2. When we call the count function, we can iterate over nums1, calculate the desired sum (tot - nums1[i]), and check the frequency of this sum in the hashmap. This way, we can count the pairs that satisfy the given sum.

When we add a value to an element in nums2, we need to update the hashmap accordingly.

Solution Code

import java.util.HashMap;
import java.util.Map;

class FindSumPairs {
    Map<Integer, Integer> map;
    int[] nums1;
    int[] nums2;

    public FindSumPairs(int[] nums1, int[] nums2) {
        this.map = new HashMap<>();
        this.nums1 = nums1;
        this.nums2 = nums2;
        
        for (int num : nums1) {
            for (int num2 : nums2) {
                int sum = num + num2;
                map.put(sum, map.getOrDefault(sum, 0) + 1);
            }
        }
    }

    public void add(int index, int val) {
        int num = nums2[index];
        for (int num1 : nums1) {
            int prevSum = num1 + num;
            int newSum = num1 + num + val;
            map.put(prevSum, map.get(prevSum) - 1);
            map.put(newSum, map.getOrDefault(newSum, 0) + 1);
        }
        nums2[index] += val;
    }

    public int count(int tot) {
        return map.getOrDefault(tot, 0);
    }
}

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