diff --git a/combination-sum/essaysir.java b/combination-sum/essaysir.java
new file mode 100644
index 0000000000..af0baba943
--- /dev/null
+++ b/combination-sum/essaysir.java
@@ -0,0 +1,32 @@
+import java.util.*;
+
+class Solution {
+	public List<List<Integer>> combinationSum(int[] candidates, int target) {
+		// 합이 target 이 되는 경우의 수를 모두 고르시오
+		// target 이 150 이하
+		List<List<Integer>> result = new ArrayList<>();
+		List<Integer> current = new ArrayList<>();
+
+		backTracking(candidates, target, 0 , result, current);
+		return result;
+	}
+
+	public void backTracking(int[] candidates, int remain, int start, List<List<Integer>> result, List<Integer> current) {
+		if ( remain == 0 ){
+			result.add(new ArrayList<>(current));
+			return;
+		}
+
+		if ( remain < 0 ){
+			return;
+		}
+
+		for (int i = start; i < candidates.length; i++) {
+			current.add(candidates[i]);
+
+			backTracking(candidates, remain - candidates[i], i , result, current);
+
+			current.remove(current.size() - 1);
+		}
+	}
+}
\ No newline at end of file
diff --git a/number-of-1-bits/essaysir.java b/number-of-1-bits/essaysir.java
new file mode 100644
index 0000000000..bc21698bf3
--- /dev/null
+++ b/number-of-1-bits/essaysir.java
@@ -0,0 +1,11 @@
+class Solution {
+    public int hammingWeight(int n) {
+        // 이진수로 바꾸고, 1 이 몇개 있는 지 파악한다.
+        return Integer.bitCount(n);
+
+        // 추가 풀이.
+        // String ans = Integer.toBinaryString(n).replace("0","");
+        // return ans.length();
+    }
+}
+
diff --git a/valid-palindrome/essaysir.java b/valid-palindrome/essaysir.java
new file mode 100644
index 0000000000..0580bda1c1
--- /dev/null
+++ b/valid-palindrome/essaysir.java
@@ -0,0 +1,19 @@
+class Solution {
+    public boolean isPalindrome(String s) {
+        StringBuilder sb = new StringBuilder();
+        for (char c : s.toCharArray()) {
+            if (Character.isLetterOrDigit(c)) {
+                sb.append(Character.toLowerCase(c));
+            }
+        }
+
+        String result = sb.toString();
+        char[] x = result.toCharArray();
+        for (int i = 0; i < x.length / 2; i++) {
+            if (x[i] != x[x.length - i - 1]) {
+                return false;
+            }
+        }
+        return true;
+    }
+}