main.go

// Source: https://leetcode.com/problems/finding-pairs-with-a-certain-sum
// Title: Finding Pairs With a Certain Sum
// Difficulty: Medium
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// You are given two integer arrays `nums1` and `nums2`. You are tasked to implement a data structure that supports queries of two types:
//
// - **Add** a positive integer to an element of a given index in the array `nums2`.
// - **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 <= 10^5`
// - `1 <= nums1[i] <= 10^9`
// - `1 <= nums2[i] <= 10^5`
// - `0 <= index < nums2.length`
// - `1 <= val <= 10^5`
// - `1 <= tot <= 10^9`
// - At most `1000` calls are made to `add` and `count` **each**.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

// Counter
//
// Note that nums1 is shorter, it is better to loop through nums1

type FindSumPairs struct {
	nums1  []int
	nums2  []int
	count2 map[int]int
}

func Constructor(nums1 []int, nums2 []int) FindSumPairs {
	count2 := make(map[int]int)
	for _, num := range nums2 {
		count2[num]++
	}

	return FindSumPairs{
		nums1:  nums1,
		nums2:  nums2,
		count2: count2,
	}
}

func (this *FindSumPairs) Add(index int, val int) {
	this.count2[this.nums2[index]]--
	this.nums2[index] += val
	this.count2[this.nums2[index]]++
}

func (this *FindSumPairs) Count(tot int) int {
	ans := 0
	for _, num := range this.nums1 {
		ans += this.count2[tot-num]
	}
	return ans
}