feat: add solution for find median from data stream

Author: bujosa

src/hard/find_median_from_data_stream.rs

@@ -1,20 +1,47 @@
 #![allow(dead_code)]
-struct MedianFinder {}
+use std::cmp::Reverse;
+use std::collections::BinaryHeap;
+
+pub struct MedianFinder {
+    max_heap: BinaryHeap<i32>,
+    min_heap: BinaryHeap<Reverse<i32>>,
+}
 
-/**
- * `&self` means the method takes an immutable reference.
- * If you need a mutable reference, change it to `&mut self` instead.
- */
 impl MedianFinder {
-    fn new() -> Self {
-        todo!()
+    pub fn new() -> Self {
+        MedianFinder {
+            max_heap: BinaryHeap::new(),
+            min_heap: BinaryHeap::new(),
+        }
     }
 
-    fn add_num(&self, num: i32) {
-        todo!()
+    pub fn add_num(&mut self, num: i32) {
+        self.max_heap.push(num);
+        if let Some(max_heap_top) = self.max_heap.pop() {
+            self.min_heap.push(Reverse(max_heap_top));
+        }
+        if self.max_heap.len() < self.min_heap.len() {
+            if let Some(Reverse(min_heap_top)) = self.min_heap.pop() {
+                self.max_heap.push(min_heap_top);
+            }
+        }
     }
 
-    fn find_median(&self) -> f64 {
-        todo!()
+    pub fn find_median(&self) -> f64 {
+        if self.max_heap.len() > self.min_heap.len() {
+            *self.max_heap.peek().unwrap() as f64
+        } else {
+            (*self.max_heap.peek().unwrap() as f64 + self.min_heap.peek().unwrap().0 as f64) / 2.0
+        }
     }
 }
+
+#[test]
+fn test_find_median_from_data_stream() {
+    let mut median_finder = MedianFinder::new();
+    median_finder.add_num(1);
+    median_finder.add_num(2);
+    assert_eq!(median_finder.find_median(), 1.5);
+    median_finder.add_num(3);
+    assert_eq!(median_finder.find_median(), 2.0);
+}