Finding Pairs With a Certain Sum | Leetcode 1865

class FindSumPairs {
    vector<int> arr1,arr2;
    unordered_map<int,int> ele_freq;
public:
    FindSumPairs(vector<int>& nums1, vector<int>& nums2) {
        arr1 = nums1;
        arr2 = nums2;
        for(int ele: nums2)
            ele_freq[ele]++;
    }
    
    void add(int index, int val) {
        ele_freq[arr2[index]]--;
        arr2[index] += val;
        ele_freq[arr2[index]]++;
    }
    
    int count(int tot) {
        int res = 0;
        for(int ele: arr1)
            if(ele_freq.count(tot-ele))
                res += ele_freq[tot-ele];
        return res;
    }
};

/**
 * Your FindSumPairs object will be instantiated and called as such:
 * FindSumPairs* obj = new FindSumPairs(nums1, nums2);
 * obj->add(index,val);
 * int param_2 = obj->count(tot);
 */
 /*
 //JAVA
 import java.util.HashMap;
import java.util.Map;

class FindSumPairs {
    private int[] arr1;
    private int[] arr2;
    private Map<Integer, Integer> eleFreq;

    public FindSumPairs(int[] nums1, int[] nums2) {
        this.arr1 = nums1;
        this.arr2 = nums2.clone();
        this.eleFreq = new HashMap<>();
        for (int ele : nums2) {
            eleFreq.put(ele, eleFreq.getOrDefault(ele, 0) + 1);
        }
    }
    
    public void add(int index, int val) {
        int oldVal = arr2[index];
        eleFreq.put(oldVal, eleFreq.get(oldVal) - 1);
        arr2[index] += val;
        int newVal = arr2[index];
        eleFreq.put(newVal, eleFreq.getOrDefault(newVal, 0) + 1);
    }
    
    public int count(int tot) {
        int res = 0;
        for (int ele : arr1) {
            int target = tot - ele;
            res += eleFreq.getOrDefault(target, 0);
        }
        return res;
    }
}

#Python
from typing import List
from collections import defaultdict

class FindSumPairs:
    def __init__(self, nums1: List[int], nums2: List[int]):
        self.arr1 = nums1
        self.arr2 = nums2.copy()
        self.ele_freq = defaultdict(int)
        for ele in nums2:
            self.ele_freq[ele] += 1
    
    def add(self, index: int, val: int) -> None:
        old_val = self.arr2[index]
        self.ele_freq[old_val] -= 1
        self.arr2[index] += val
        new_val = self.arr2[index]
        self.ele_freq[new_val] += 1
    
    def count(self, tot: int) -> int:
        res = 0
        for ele in self.arr1:
            target = tot - ele
            res += self.ele_freq.get(target, 0)
        return res
*/

just an improve, you can do hash on nums1 too , i think it can be faster y there are a lot of repeated values on num1..

class FindSumPairs:

def __init__(self, nums1: List[int], nums2: List[int]):
    
    self.val_nums1 = {}
    for n1 in nums1:
        if n1 in self.val_nums1:
            self.val_nums1[n1] += 1
        else:
            self.val_nums1[n1] = 1

    self.nums2 = nums2
    self.val_nums2 = {}
    for n2 in self.nums2:
        if n2 in self.val_nums2:
            self.val_nums2[n2] +=1
        else:
            self.val_nums2[n2] = 1

def add(self, index: int, val: int) -> None:
        self.val_nums2[self.nums2[index]] -=1
        self.nums2[index] += val
        if self.nums2[index] in self.val_nums2:
            self.val_nums2[self.nums2[index]] +=1
        else:
            self.val_nums2[self.nums2[index]] = 1

def count(self, tot: int) -> int:
    ans = 0
    for n1,count_n1 in self.val_nums1.items():
        need_n2 = tot-n1
        if need_n2 in self.val_nums2:
            ans += count_n1*self.val_nums2[need_n2]
    return ans