problems(rs): add “501. Find Mode in Binary Search Tree”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/rs/find-mode-in-binary-search-tree.rs

@@ -0,0 +1,54 @@
+use std::cell::RefCell;
+use std::collections::BTreeMap;
+use std::rc::Rc;
+
+// Definition for a binary tree node.
+// #[derive(Debug, PartialEq, Eq)]
+// pub struct TreeNode {
+//     pub val: i32,
+//     pub left: Option<Rc<RefCell<TreeNode>>>,
+//     pub right: Option<Rc<RefCell<TreeNode>>>,
+// }
+
+// impl TreeNode {
+//     #[inline]
+//     pub fn new(val: i32) -> Self {
+//         TreeNode {
+//             val,
+//             left: None,
+//             right: None,
+//         }
+//     }
+// }
+
+// struct Solution {}
+impl Solution {
+    pub fn find_mode(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
+        let mut counters: BTreeMap<i32, usize> = BTreeMap::new();
+
+        let mut stack: Vec<Option<Rc<RefCell<TreeNode>>>> = Vec::new();
+        stack.push(root.clone());
+
+        while let Some(node) = stack.pop() {
+            if node.is_none() {
+                continue;
+            }
+
+            let node = node.unwrap();
+            let n = node.borrow();
+
+            counters
+                .entry(n.val)
+                .and_modify(|curr| *curr += 1)
+                .or_insert(1);
+            stack.push(n.left.clone());
+            stack.push(n.right.clone());
+        }
+
+        let maximum = *counters.values().max().unwrap();
+        counters
+            .iter()
+            .filter_map(|(&k, &c)| if c == maximum { Some(k) } else { None })
+            .collect()
+    }
+}