diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index 863cef1..489a876 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -609,3 +609,5 @@ mod p2176_count_equal_and_divisible_pairs_in_an_array;
 mod p2364_count_number_of_bad_pairs;
 
 mod p2563_count_the_number_of_fair_pairs;
+
+mod p781_rabbits_in_forest;
diff --git a/src/problem/p2563_count_the_number_of_fair_pairs.rs b/src/problem/p2563_count_the_number_of_fair_pairs.rs
index edcb0d0..22cc2c1 100644
--- a/src/problem/p2563_count_the_number_of_fair_pairs.rs
+++ b/src/problem/p2563_count_the_number_of_fair_pairs.rs
@@ -14,7 +14,7 @@ impl Solution {
             let l = nums[0..i].partition_point(|&x| x + nums[i] < lower);
             let r = nums[0..i].partition_point(|&x| x + nums[i] <= upper);
 
-            result += (r - l);
+            result += r - l;
         }
 
         result as i64
diff --git a/src/problem/p781_rabbits_in_forest.rs b/src/problem/p781_rabbits_in_forest.rs
new file mode 100644
index 0000000..e521e3e
--- /dev/null
+++ b/src/problem/p781_rabbits_in_forest.rs
@@ -0,0 +1,46 @@
+/**
+ * [781] Rabbits in Forest
+ */
+pub struct Solution {}
+
+// submission codes start here
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn num_rabbits(answers: Vec<i32>) -> i32 {
+        let mut map = HashMap::new();
+
+        for i in answers.into_iter() {
+            let entry = map.entry(i).or_insert(0);
+            *entry += 1;
+        }
+
+        let mut result = 0;
+
+        for (k, mut v) in map.into_iter() {
+            // 同种颜色兔子最多有k + 1个
+            let max_count = k + 1;
+
+            while v > 0 {
+                result += max_count;
+                v -= max_count;
+            }
+        }
+
+        result
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_781() {
+        assert_eq!(5, Solution::num_rabbits(vec![1, 0, 1, 0, 0]));
+        assert_eq!(5, Solution::num_rabbits(vec![1, 1, 2]));
+        assert_eq!(11, Solution::num_rabbits(vec![10, 10, 10]))
+    }
+}