diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index ca6ff78..e94d0fc 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -386,3 +386,5 @@ mod p3138_minimum_length_of_anagram_concatenation;
 mod p2545_sort_the_students_by_their_kth_score;
 
 mod p1387_sort_integers_by_the_power_value;
+
+mod p855_exam_room;
diff --git a/src/problem/p855_exam_room.rs b/src/problem/p855_exam_room.rs
new file mode 100644
index 0000000..dbbabc0
--- /dev/null
+++ b/src/problem/p855_exam_room.rs
@@ -0,0 +1,154 @@
+/**
+ * [855] Exam Room
+ */
+pub struct Solution {}
+
+// submission codes start here
+use std::cmp::Ordering;
+use std::collections::{BTreeSet, BinaryHeap};
+
+#[derive(Debug, Eq, PartialEq)]
+struct Interval {
+    start: i32,
+    end: i32,
+}
+
+impl Interval {
+    fn new(start: i32, end: i32) -> Self {
+        Self { start, end }
+    }
+
+    fn distance(&self) -> i32 {
+        (self.end - self.start) / 2
+    }
+}
+
+impl Ord for Interval {
+    fn cmp(&self, other: &Self) -> Ordering {
+        // Interval将会被放入BinaryHeap中进行排序
+        // 所以先返回middle的大
+        // 再返回start小的
+        match self.distance().cmp(&other.distance()) {
+            Ordering::Equal => other.start.cmp(&self.start),
+            r => r,
+        }
+    }
+}
+
+impl PartialOrd for Interval {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(&other))
+    }
+}
+
+struct ExamRoom {
+    n: i32,
+    seats: BTreeSet<i32>,
+    intervals: BinaryHeap<Interval>,
+}
+
+/**
+ * `&self` means the method takes an immutable reference.
+ * If you need a mutable reference, change it to `&mut self` instead.
+ */
+impl ExamRoom {
+    fn new(n: i32) -> Self {
+        Self {
+            n,
+            seats: BTreeSet::new(),
+            intervals: BinaryHeap::new(),
+        }
+    }
+
+    fn seat(&mut self) -> i32 {
+        if self.seats.is_empty() {
+            self.seats.insert(0);
+            return 0;
+        }
+
+        // 坐在最左边到左边同学的距离
+        let left = *self.seats.iter().next().unwrap();
+        let right = self.n - 1 - *self.seats.iter().rev().next().unwrap();
+
+        // 当当前坐着的同学数量大于等于2
+        while self.seats.len() >= 2 {
+            if let Some(interval) = self.intervals.peek() {
+                // 判断是否需要延迟删除该区间
+                if !self.seats.contains(&interval.start)
+                    || !self.seats.contains(&interval.end)
+                    || *self.seats.range((interval.start + 1)..).next().unwrap() != interval.end
+                {
+                    self.intervals.pop();
+                    continue;
+                }
+
+                // 判断坐在区间的中间是否比坐在两个端点更好
+                let distance = interval.distance();
+                // 小于等于 left 是因为当距离一致时坐在较小的地方
+                if distance < right || distance <= left {
+                    break;
+                }
+
+                // 选择坐在中间
+                let interval = self.intervals.pop().unwrap();
+                let middle = interval.start + distance;
+                self.intervals.push(Interval::new(interval.start, middle));
+                self.intervals.push(Interval::new(middle, interval.end));
+                self.seats.insert(middle);
+                return middle;
+            }
+        }
+
+        // 只有一个同学
+        if right > left {
+            let last = *self.seats.last().unwrap();
+            self.intervals.push(Interval::new(last, self.n - 1));
+            self.seats.insert(self.n - 1);
+            self.n - 1
+        } else {
+            let first = *self.seats.first().unwrap();
+            self.intervals.push(Interval::new(0, first));
+            self.seats.insert(0);
+            0
+        }
+    }
+
+    fn leave(&mut self, p: i32) {
+        // 如果学生不是最左或者是最右的区间
+        // 那么删除会产生新的区间
+        // 判断区间是否有效是Self::seat()的工作
+        if p != *self.seats.first().unwrap() && p != *self.seats.last().unwrap() {
+            let pre = *self.seats.range(..p).last().unwrap();
+            let next = *self.seats.range((p + 1)..).next().unwrap();
+
+            self.intervals.push(Interval::new(pre, next));
+        }
+
+        self.seats.remove(&p);
+    }
+}
+
+/**
+ * Your ExamRoom object will be instantiated and called as such:
+ * let obj = ExamRoom::new(n);
+ * let ret_1: i32 = obj.seat();
+ * obj.leave(p);
+ */
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_855() {
+        let mut room = ExamRoom::new(10);
+        assert_eq!(0, room.seat());
+        assert_eq!(9, room.seat());
+        assert_eq!(4, room.seat());
+        assert_eq!(2, room.seat());
+        room.leave(4);
+        assert_eq!(5, room.seat());
+    }
+}