feat: add rust solution to lc problem: No.1865 (#4550)

No.1865.Finding Pairs With a Certain Sum

Author: yanglbme

solution/1800-1899/1865.Finding Pairs With a Certain Sum/README.md

@@ -273,6 +273,52 @@ class FindSumPairs {
  */
 ```
 
+#### Rust
+
+```rust
+use std::collections::HashMap;
+
+struct FindSumPairs {
+    nums1: Vec<i32>,
+    nums2: Vec<i32>,
+    cnt: HashMap<i32, i32>,
+}
+
+impl FindSumPairs {
+    fn new(nums1: Vec<i32>, nums2: Vec<i32>) -> Self {
+        let mut cnt = HashMap::new();
+        for &x in &nums2 {
+            *cnt.entry(x).or_insert(0) += 1;
+        }
+        Self { nums1, nums2, cnt }
+    }
+
+    fn add(&mut self, index: i32, val: i32) {
+        let i = index as usize;
+        let old_val = self.nums2[i];
+        *self.cnt.entry(old_val).or_insert(0) -= 1;
+        if self.cnt[&old_val] == 0 {
+            self.cnt.remove(&old_val);
+        }
+
+        self.nums2[i] += val;
+        let new_val = self.nums2[i];
+        *self.cnt.entry(new_val).or_insert(0) += 1;
+    }
+
+    fn count(&self, tot: i32) -> i32 {
+        let mut ans = 0;
+        for &x in &self.nums1 {
+            let target = tot - x;
+            if let Some(&c) = self.cnt.get(&target) {
+                ans += c;
+            }
+        }
+        ans
+    }
+}
+```
+
 #### JavaScript
 
 ```js

solution/1800-1899/1865.Finding Pairs With a Certain Sum/README_EN.md

@@ -271,6 +271,52 @@ class FindSumPairs {
  */
 ```
 
+#### Rust
+
+```rust
+use std::collections::HashMap;
+
+struct FindSumPairs {
+    nums1: Vec<i32>,
+    nums2: Vec<i32>,
+    cnt: HashMap<i32, i32>,
+}
+
+impl FindSumPairs {
+    fn new(nums1: Vec<i32>, nums2: Vec<i32>) -> Self {
+        let mut cnt = HashMap::new();
+        for &x in &nums2 {
+            *cnt.entry(x).or_insert(0) += 1;
+        }
+        Self { nums1, nums2, cnt }
+    }
+
+    fn add(&mut self, index: i32, val: i32) {
+        let i = index as usize;
+        let old_val = self.nums2[i];
+        *self.cnt.entry(old_val).or_insert(0) -= 1;
+        if self.cnt[&old_val] == 0 {
+            self.cnt.remove(&old_val);
+        }
+
+        self.nums2[i] += val;
+        let new_val = self.nums2[i];
+        *self.cnt.entry(new_val).or_insert(0) += 1;
+    }
+
+    fn count(&self, tot: i32) -> i32 {
+        let mut ans = 0;
+        for &x in &self.nums1 {
+            let target = tot - x;
+            if let Some(&c) = self.cnt.get(&target) {
+                ans += c;
+            }
+        }
+        ans
+    }
+}
+```
+
 #### JavaScript
 
 ```js

solution/1800-1899/1865.Finding Pairs With a Certain Sum/Solution.rs

@@ -0,0 +1,41 @@
+use std::collections::HashMap;
+
+struct FindSumPairs {
+    nums1: Vec<i32>,
+    nums2: Vec<i32>,
+    cnt: HashMap<i32, i32>,
+}
+
+impl FindSumPairs {
+    fn new(nums1: Vec<i32>, nums2: Vec<i32>) -> Self {
+        let mut cnt = HashMap::new();
+        for &x in &nums2 {
+            *cnt.entry(x).or_insert(0) += 1;
+        }
+        Self { nums1, nums2, cnt }
+    }
+
+    fn add(&mut self, index: i32, val: i32) {
+        let i = index as usize;
+        let old_val = self.nums2[i];
+        *self.cnt.entry(old_val).or_insert(0) -= 1;
+        if self.cnt[&old_val] == 0 {
+            self.cnt.remove(&old_val);
+        }
+
+        self.nums2[i] += val;
+        let new_val = self.nums2[i];
+        *self.cnt.entry(new_val).or_insert(0) += 1;
+    }
+
+    fn count(&self, tot: i32) -> i32 {
+        let mut ans = 0;
+        for &x in &self.nums1 {
+            let target = tot - x;
+            if let Some(&c) = self.cnt.get(&target) {
+                ans += c;
+            }
+        }
+        ans
+    }
+}