diff --git a/combination-sum/Ujoonnee.java b/combination-sum/Ujoonnee.java
new file mode 100644
index 000000000..787cc32fb
--- /dev/null
+++ b/combination-sum/Ujoonnee.java
@@ -0,0 +1,28 @@
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Stack;
+
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> output = new ArrayList<>();
+        Stack<Integer> nums = new Stack<>();
+        dfs(candidates, output, nums, target, 0, 0);
+
+        return output;
+    }
+
+    private void dfs(int[] candidates, List<List<Integer>> output, Stack<Integer> nums, int target, int start, int total) {
+        if (total > target) {
+            return;
+        }
+        if (total == target) {
+            output.add(new ArrayList<>(nums));
+        }
+        for (int i=start; i<candidates.length; i++) {
+            int num = candidates[i];
+            nums.push(num);
+            dfs(candidates, output, nums, target, i, total + num);
+            nums.pop();
+        }
+    }
+}
diff --git a/number-of-1-bits/Ujoonnee.java b/number-of-1-bits/Ujoonnee.java
new file mode 100644
index 000000000..ef8bbad53
--- /dev/null
+++ b/number-of-1-bits/Ujoonnee.java
@@ -0,0 +1,19 @@
+class Solution {
+    public int hammingWeight(int n) {
+        int answer = 0;
+        while (n > 0) {
+            answer += n & 1;
+            n >>= 1;
+        }
+
+        return answer;
+    }
+}
+
+/*
+class Solution {
+    public int hammingWeight(int n) {
+        return Integer.toBinaryString(n).replace("0", "").length();
+    }
+}
+*/
diff --git a/valid-palindrome/Ujoonnee.java b/valid-palindrome/Ujoonnee.java
new file mode 100644
index 000000000..0270534da
--- /dev/null
+++ b/valid-palindrome/Ujoonnee.java
@@ -0,0 +1,51 @@
+// time complexity : O(n)
+// space complexity : O(1)
+
+class Solution {
+    public boolean isPalindrome(String s) {
+        s = s.toLowerCase();
+        int i = 0;
+        int j = s.length() - 1;
+        while(i < j) {
+            char c1 = s.charAt(i);
+            if (!isAlphanumeric(c1)) {
+                i++;
+                continue;
+            }
+
+            char c2 = s.charAt(j);
+            if (!isAlphanumeric(c2)) {
+                j--;
+                continue;
+            }
+
+            if (c1 != c2) {
+                return false;
+            }
+            i++;
+            j--;
+        }
+
+        return true;
+    }
+
+    private boolean isAlphanumeric(char c) {
+        return (c >= '0' && c<='9') || (c >= 'A' && c <= 'Z') || (c >= 'a' && c<= 'z');
+    }
+}
+
+/*
+class Solution {
+    public boolean isPalindrome(String s) {
+        s = s.toLowerCase();
+        String[] split = s.replaceAll("[^A-Za-z0-9]", "").split("");
+        for (int i=0; i<split.length/2; i++) {
+            if (!split[i].equals(split[split.length-1-i])) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
+*/