diff --git a/solution/0500-0599/0500.Keyboard Row/Solution.java b/solution/0500-0599/0500.Keyboard Row/Solution.java
new file mode 100644
index 0000000000000..cf4cbd85e342b
--- /dev/null
+++ b/solution/0500-0599/0500.Keyboard Row/Solution.java	
@@ -0,0 +1,34 @@
+class Solution {
+
+    public String[] findWords(String[] words) {
+        if (words == null) {
+            return null;
+        }
+        ArrayList<String> list = new ArrayList<>();
+        String[] keyboards = {"qwertyuiop", "asdfghjkl", "zxcvbnm"};
+        for (int i = 0; i < words.length; i++) {
+            String word = words[i].toLowerCase();
+            for (int j = 0; j < keyboards.length; j++) {
+                // 先用word首字符确定属于哪一行
+                if (keyboards[j].indexOf(word.charAt(0)) > -1) {
+                    // 判断word字符串所有字符是否都属于同一行
+                    boolean match = match(keyboards[j], word, list);
+                    if (match) {
+                        list.add(words[i]);
+                    }
+                    break;
+                }
+            }
+        }
+        return list.toArray(new String[list.size()]);
+    }
+
+    private boolean match(String keyboard, String word, ArrayList<String> list) {
+        for (int i = 1; i < word.length(); i++) {
+            if (keyboard.indexOf(word.charAt(i)) < 0) {
+                return false;
+            }
+        }
+        return true;
+    }
+}
diff --git a/solution/0500-0599/0501.Find Mode in Binary Search Tree/Solution.java b/solution/0500-0599/0501.Find Mode in Binary Search Tree/Solution.java
new file mode 100644
index 0000000000000..c1b05e8549886
--- /dev/null
+++ b/solution/0500-0599/0501.Find Mode in Binary Search Tree/Solution.java	
@@ -0,0 +1,45 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    int max = 0;
+    int cur = 0;
+    TreeNode preNode = null;
+
+    public int[] findMode(TreeNode root) {
+        ArrayList<Integer> list = new ArrayList<>();
+        findMode(root, list);
+        int[] res = new int[list.size()];
+        for (int i = 0; i < list.size(); i++) {
+            res[i] = list.get(i);
+        }
+        return res;
+    }
+
+    private void findMode(TreeNode root, ArrayList<Integer> list) {
+        if (root == null) {
+            return;
+        }
+        findMode(root.left, list);
+        if (preNode != null && root.val == preNode.val) {
+            cur++;
+        } else {
+            cur = 1;
+        }
+        if (max < cur) {
+            max = cur;
+            list.clear();
+            list.add(root.val);
+        } else if (max == cur) {
+            list.add(root.val);
+        }
+        preNode = root;
+        findMode(root.right, list);
+    }
+}