diff --git a/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/README.md b/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/README.md
index 17f3b3006..c0a9548e1 100755
--- a/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/README.md
+++ b/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/README.md
@@ -1,28 +1,36 @@
 # [1865.Finding Pairs With a Certain Sum][title]
 
-> [!WARNING|style:flat]
-> This question is temporarily unanswered if you have good ideas. Welcome to [Create Pull Request PR](https://github.com/kylesliu/awesome-golang-algorithm)
-
 ## Description
+You are given two integer arrays `nums1` and `nums2`. You are tasked to implement a data structure that supports queries of two types:
 
-**Example 1:**
+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`).
 
-```
-Input: a = "11", b = "1"
-Output: "100"
-```
+Implement the `FindSumPairs` class:
 
-## 题意
-> ...
+- `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`.
 
-## 题解
+**Example 1:**
 
-### 思路1
-> ...
-Finding Pairs With a Certain Sum
-```go
 ```
-
+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
+```
 
 ## 结语
 
diff --git a/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/Solution.go b/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/Solution.go
index d115ccf5e..173e612fb 100644
--- a/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/Solution.go
+++ b/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/Solution.go
@@ -1,5 +1,58 @@
 package Solution
 
-func Solution(x bool) bool {
-	return x
+type FindSumPairs struct {
+	c1, c2 map[int]int
+	n2     []int
+}
+
+func Constructor(nums1 []int, nums2 []int) FindSumPairs {
+	f := FindSumPairs{
+		c1: map[int]int{}, c2: map[int]int{}, n2: nums2,
+	}
+	for _, n := range nums1 {
+		f.c1[n]++
+	}
+	for _, n := range nums2 {
+		f.c2[n]++
+	}
+	return f
+}
+
+func (this *FindSumPairs) Add(index int, val int) {
+	source := this.n2[index]
+	this.n2[index] += val
+	this.c2[source]--
+	if this.c2[source] == 0 {
+		delete(this.c2, source)
+	}
+	this.c2[this.n2[index]]++
+}
+
+func (this *FindSumPairs) Count(tot int) int {
+	ans := 0
+	for k, c := range this.c1 {
+		if c1, ok := this.c2[tot-k]; ok {
+			ans += c * c1
+		}
+	}
+	return ans
+}
+
+type op struct {
+	name byte
+	a, b int
+}
+
+func Solution(nums1, nums2 []int, ops []op) []int {
+	var ans []int
+	c := Constructor(nums1, nums2)
+	for _, o := range ops {
+		if o.name == 'a' {
+			c.Add(o.a, o.b)
+			continue
+		}
+		ans = append(ans, c.Count(o.a))
+	}
+	return ans
+
 }
diff --git a/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/Solution_test.go b/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/Solution_test.go
index 14ff50eb4..6eceeec53 100644
--- a/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/Solution_test.go
+++ b/leetcode/1801-1900/1865.Finding-Pairs-With-a-Certain-Sum/Solution_test.go
@@ -9,31 +9,52 @@ import (
 func TestSolution(t *testing.T) {
 	//	测试用例
 	cases := []struct {
-		name   string
-		inputs bool
-		expect bool
+		name         string
+		nums1, nums2 []int
+		ops          []op
+		expect       []int
 	}{
-		{"TestCase", true, true},
-		{"TestCase", true, true},
-		{"TestCase", false, false},
+		{"TestCase1", []int{1, 1, 2, 2, 2, 3}, []int{1, 4, 5, 2, 5, 4}, []op{
+			{
+				name: 'c', a: 7,
+			},
+			{
+				name: 'a', a: 3, b: 2,
+			},
+			{
+				name: 'c', a: 8,
+			},
+			{
+				name: 'c', a: 4,
+			},
+			{
+				name: 'a', a: 0, b: 1,
+			},
+			{
+				name: 'a', a: 1, b: 1,
+			},
+			{
+				name: 'c', a: 7,
+			},
+		}, []int{8, 2, 1, 11}},
 	}
 
 	//	开始测试
 	for i, c := range cases {
 		t.Run(c.name+" "+strconv.Itoa(i), func(t *testing.T) {
-			got := Solution(c.inputs)
+			got := Solution(c.nums1, c.nums2, c.ops)
 			if !reflect.DeepEqual(got, c.expect) {
-				t.Fatalf("expected: %v, but got: %v, with inputs: %v",
-					c.expect, got, c.inputs)
+				t.Fatalf("expected: %v, but got: %v, with inputs: %v %v %v",
+					c.expect, got, c.nums1, c.nums2, c.ops)
 			}
 		})
 	}
 }
 
-//	压力测试
+// 压力测试
 func BenchmarkSolution(b *testing.B) {
 }
 
-//	使用案列
+// 使用案列
 func ExampleSolution() {
 }