diff --git a/501_Find_Mode_in_Binary_Search_Tree.java b/501_Find_Mode_in_Binary_Search_Tree.java
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+++ b/501_Find_Mode_in_Binary_Search_Tree.java
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+// id: 501
+// Name: Find Mode in Binary Search Tree
+// link: https://leetcode.com/problems/find-mode-in-binary-search-tree/
+// Difficulty: Easy
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+
+    Map<Integer, Integer> map = new HashMap<>();
+
+    public int[] findMode(TreeNode root) {
+        dfs(root);
+
+        // get max from node;
+        int max = 0;
+        int frequency = 0;
+        for (int key: map.keySet()) {
+            if (map.get(key) > max) {
+                max = map.get(key);
+                frequency = 1;
+            }
+            else if (map.get(key) == max) {
+                frequency++;
+            }
+        }
+
+        // get all nodes with frequency = max
+        int [] ans = new int[frequency];
+        int i = 0;
+
+        for (int key: map.keySet()) {
+            if (map.get(key) == max) {
+                ans[i++] = key; 
+            }
+        }
+
+
+        return ans;
+    }
+
+    private void dfs(TreeNode node) {
+        if (node== null) return;
+
+        dfs(node.left);
+        dfs(node.right);
+        map.put(node.val, map.getOrDefault(node.val, 0)+1);
+    }
+}