LeetCode-in-Java

1865. Finding Pairs With a Certain Sum

Medium

You are given two integer arrays nums1 and nums2. You are tasked to implement a data structure that supports queries of two types:

1. Add a positive integer to an element of a given index in the array nums2.
2. Count the number of pairs (i, j) such that nums1[i] + nums2[j] equals a given value (0 <= i < nums1.length and 0 <= j < nums2.length).

Implement the FindSumPairs class:

• FindSumPairs(int[] nums1, int[] nums2) Initializes the FindSumPairs object with two integer arrays nums1 and nums2.
• void add(int index, int val) Adds val to nums2[index], i.e., apply nums2[index] += val.
• int count(int tot) Returns the number of pairs (i, j) such that nums1[i] + nums2[j] == tot.

Example 1:

Input [“FindSumPairs”, “count”, “add”, “count”, “count”, “add”, “add”, “count”] [[[1, 1, 2, 2, 2, 3], [1, 4, 5, 2, 5, 4]], [7], [3, 2], [8], [4], [0, 1], [1, 1], [7]]

Output: [null, 8, null, 2, 1, null, null, 11]

Explanation:

FindSumPairs findSumPairs = new FindSumPairs([1, 1, 2, 2, 2, 3], [1, 4, 5, 2, 5, 4]);

findSumPairs.count(7); // return 8; pairs (2,2), (3,2), (4,2), (2,4), (3,4), (4,4) make 2 + 5 and pairs (5,1), (5,5) make 3 + 4

findSumPairs.add(3, 2); // now nums2 = [1,4,5,4,5,4]

findSumPairs.count(8); // return 2; pairs (5,2), (5,4) make 3 + 5

findSumPairs.count(4); // return 1; pair (5,0) makes 3 + 1

findSumPairs.add(0, 1); // now nums2 = [2,4,5,4,5,4]

findSumPairs.add(1, 1); // now nums2 = [2,5,5,4,5,4]

findSumPairs.count(7); // return 11; pairs (2,1), (2,2), (2,4), (3,1), (3,2), (3,4), (4,1), (4,2), (4,4) make 2 + 5 and pairs (5,3), (5,5) make 3 + 4

Constraints:

• 1 <= nums1.length <= 1000
• 1 <= nums2.length <= 105
• 1 <= nums1[i] <= 109
• 1 <= nums2[i] <= 105
• 0 <= index < nums2.length
• 1 <= val <= 105
• 1 <= tot <= 109
• At most 1000 calls are made to add and count each.

Solution

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

public class FindSumPairs {
    private Map<Integer, Integer> numFreq = new HashMap<>();
    private int[] nums1;
    private int[] nums2;

    public FindSumPairs(int[] nums1, int[] nums2) {
        this.nums1 = nums1;
        this.nums2 = nums2;
        for (int num : nums2) {
            numFreq.put(num, numFreq.getOrDefault(num, 0) + 1);
        }
    }

    public void add(int index, int val) {
        numFreq.put(nums2[index], numFreq.getOrDefault(nums2[index], 0) - 1);
        nums2[index] += val;
        numFreq.put(nums2[index], numFreq.getOrDefault(nums2[index], 0) + 1);
    }

    public int count(int tot) {
        int res = 0;
        for (int num : nums1) {
            res += numFreq.getOrDefault(tot - num, 0);
        }
        return res;
    }
}

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