diff --git a/combination-sum/hi-rachel.py b/combination-sum/hi-rachel.py
new file mode 100644
index 000000000..abad32ddc
--- /dev/null
+++ b/combination-sum/hi-rachel.py
@@ -0,0 +1,20 @@
+# DFS + 백트래킹
+# 중복 조합 문제
+# O(2^t) time, O(재귀 깊이 (t) + 결과 조합의 수 (len(result))) space
+
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        nums, result = [], []
+        def dfs(start, total):
+            if total > target:
+                return
+            if total == target:
+                result.append(nums[:])
+            for i in range(start, len(candidates)):
+                num = candidates[i]
+                nums.append(num)
+                dfs(i, total + num)
+                nums.pop()
+
+        dfs(0, 0)
+        return result
diff --git a/decode-ways/hi-rachel.py b/decode-ways/hi-rachel.py
new file mode 100644
index 000000000..a1d684eb1
--- /dev/null
+++ b/decode-ways/hi-rachel.py
@@ -0,0 +1,19 @@
+# 디코드 가능 1 ~ 26
+# 숫자의 첫 번째 자리가 0이라면 decode x
+# O(n) time, O(n) space
+
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        memo = {len(s): 1} # 문자열 끝에 도달했을 때는 경우의 수 1
+
+        def dfs(start):
+            if start in memo: # 이미 계산한 위치 재계산 x
+                return memo[start]
+            if s[start] == "0":
+                memo[start] = 0
+            elif start + 1 < len(s) and int(s[start:start + 2]) < 27: # 두 자리로 해석 가능할 때
+                memo[start] = dfs(start + 1) + dfs(start + 2) # 첫 한 자리만 decode 경우 + 두 자리 한꺼번에 decode 경우
+            else:
+                memo[start] = dfs(start + 1) # 두 자리로 decode 불가능할 때 -> 한 자리만 decode
+            return memo[start]
+        return dfs(0)
diff --git a/maximum-subarray/hi-rachel.py b/maximum-subarray/hi-rachel.py
new file mode 100644
index 000000000..b762e582f
--- /dev/null
+++ b/maximum-subarray/hi-rachel.py
@@ -0,0 +1,35 @@
+# 최대 부분 배열 합 문제
+
+# O(n^2) time, O(1) space
+# Time Limit Exceeded
+
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:
+        max_total = nums[0]
+        for i in range(len(nums)):
+            total = 0
+            for j in range(i, len(nums)):
+                total += nums[j]
+                max_total = max(total, max_total)
+        return max_total
+    
+# 개선 풀이
+# O(n) time, O(1) space
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:
+        max_total = nums[0]
+        total = nums[0]
+        for i in range(1, len(nums)):
+            total = max(nums[i], total + nums[i])
+            max_total = max(total, max_total)
+        return max_total 
+
+# DP 풀이
+# O(n) time, O(n) space
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:
+        dp = [0] * len(nums)
+        dp[0] = nums[0]
+        for i in range(1, len(nums)):
+            dp[i] = max(nums[i], dp[i - 1] + nums[i])
+        return max(dp)
diff --git a/number-of-1-bits/hi-rachel.py b/number-of-1-bits/hi-rachel.py
new file mode 100644
index 000000000..16f13b6e8
--- /dev/null
+++ b/number-of-1-bits/hi-rachel.py
@@ -0,0 +1,26 @@
+# O(log n) time, O(1) space
+# % 나머지, // 몫 
+
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        cnt = 0
+
+        while n > 0:
+            if (n % 2) == 1:
+                cnt += 1
+            n //= 2
+
+        return cnt
+    
+
+# O(log n) time, O(log n) space
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        return bin(n).count("1")
+    
+
+# TS 풀이
+# O(log n) time, O(log n) space
+# function hammingWeight(n: number): number {
+#     return n.toString(2).split('').filter(bit => bit === '1').length;
+# };
diff --git a/valid-palindrome/hi-rachel.py b/valid-palindrome/hi-rachel.py
new file mode 100644
index 000000000..c509fab9a
--- /dev/null
+++ b/valid-palindrome/hi-rachel.py
@@ -0,0 +1,20 @@
+# O(n) time, O(1) space
+# isalnum() -> 문자열이 영어, 한글 혹은 숫자로 되어있으면 참 리턴, 아니면 거짓 리턴.
+
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        l = 0
+        r = len(s) - 1
+
+        while l < r:
+            if not s[l].isalnum():
+                l += 1
+            elif not s[r].isalnum():
+                r -= 1
+            elif s[l].lower() == s[r].lower():
+                l += 1
+                r -= 1
+            else:
+                return False
+        return True
+
diff --git a/valid-palindrome/hi-rachel.ts b/valid-palindrome/hi-rachel.ts
new file mode 100644
index 000000000..c020ef1a7
--- /dev/null
+++ b/valid-palindrome/hi-rachel.ts
@@ -0,0 +1,21 @@
+// O(n) time, O(1) space
+
+function isPalindrome(s: string): boolean {
+  let low = 0,
+    high = s.length - 1;
+
+  while (low < high) {
+    while (low < high && !s[low].match(/[0-9a-zA-Z]/)) {
+      low++;
+    }
+    while (low < high && !s[high].match(/[0-9a-zA-Z]/)) {
+      high--;
+    }
+    if (s[low].toLowerCase() !== s[high].toLowerCase()) {
+      return false;
+    }
+    low++;
+    high--;
+  }
+  return true;
+}