diff --git a/3sum/ymir0804.java b/3sum/ymir0804.java
new file mode 100644
index 000000000..240420d67
--- /dev/null
+++ b/3sum/ymir0804.java
@@ -0,0 +1,39 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        List<List<Integer>> answer = new ArrayList<>();
+        Arrays.sort(nums);
+        int sum = 0;
+
+        for (int i = 0; i < nums.length-2; i++) {
+            int left = i + 1;
+            int right = nums.length -1;
+            if(i > 0 && nums[i-1] == nums[i]) {
+                continue;
+            }
+            while (left < right ) {
+                sum = nums[i] + nums[left] + nums[right];
+                if(sum > 0) {
+                    right--;
+                } else if(sum < 0 ) {
+                    left++;
+                } else {
+                    answer.add(Arrays.asList(nums[i], nums[left], nums[right]));
+                    while (left < right && nums[left] == nums[left + 1]) {
+                        left++;
+                    }
+                    while (left < right && nums[right] == nums[right - 1]) {
+                        right--;
+                    }
+                    left++;
+                    right--;
+                }
+            }
+
+        }
+        return answer;
+    }
+}
diff --git a/validate-binary-search-tree/ymir0804.java b/validate-binary-search-tree/ymir0804.java
new file mode 100644
index 000000000..18d46167a
--- /dev/null
+++ b/validate-binary-search-tree/ymir0804.java
@@ -0,0 +1,16 @@
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return isValid(root, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+    public boolean isValid(TreeNode node, long min, long max) {
+        if (node == null) {
+            return true;
+        }
+        if (node.val <= min || node.val >= max) {
+            return false;
+        }
+        return isValid(node.left, min, node.val) && isValid(node.right, node.val, max);
+    }
+
+}
+