diff --git a/1-100q/12.py b/1-100q/12.py
new file mode 100644
index 0000000..d1208e8
--- /dev/null
+++ b/1-100q/12.py
@@ -0,0 +1,25 @@
+# 1 -> "I"
+# 2 -> "II"
+# 3 -> "III"
+# 4 -> "IV"
+# 5 -> "V"
+# 1499 -> "MCDXCIX"
+
+
+class Solution:
+    def intToRoman(self, num: int) -> str:
+        nums = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1]
+        letters = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I']
+        result = ""
+        i = 0
+        
+        while num > 0:
+            times = num // nums[i]
+            
+            result += letters[i] * times
+            num -= nums[i] * times
+            
+            i += 1
+        
+        return result
+        
\ No newline at end of file
diff --git a/1-100q/13.py b/1-100q/13.py
new file mode 100644
index 0000000..aafd3a4
--- /dev/null
+++ b/1-100q/13.py
@@ -0,0 +1,28 @@
+class Solution:
+    def romanToInt(self, s: str) -> int:
+        letter = {
+            "I": 1,
+            "IV": 4,
+            "V": 5,
+            "IX": 9,
+            "X": 10,
+            "XL": 40,
+            "L": 50,
+            "XC": 90,
+            "C": 100,
+            "CD": 400,
+            "D": 500,
+            "CM": 900,
+            "M": 1000,
+        }
+        result = 0
+        
+        while s:
+            if s[0:2] in letter:
+                result += letter.get(s[0:2])
+                s = s[2:]
+            else:
+                result += letter.get(s[0])
+                s = s[1:]
+        
+        return result
\ No newline at end of file
diff --git a/1-100q/20.py b/1-100q/20.py
new file mode 100644
index 0000000..21797ae
--- /dev/null
+++ b/1-100q/20.py
@@ -0,0 +1,18 @@
+class Solution:
+    def isValid(self, s: str) -> bool:
+        parentheses = {
+            "(": ")",
+            "{": "}",
+            "[": "]",
+        }
+        stack = []
+        
+        for char in s:
+            if char in parentheses:
+                stack.append(char)
+            elif stack and parentheses[stack[-1]] == char:
+                stack.pop()
+            else:
+                return False
+        
+        return not stack
\ No newline at end of file
diff --git a/1-100q/27.py b/1-100q/27.py
new file mode 100644
index 0000000..b7ae672
--- /dev/null
+++ b/1-100q/27.py
@@ -0,0 +1,16 @@
+class Solution:
+    def removeElement(self, nums, val):
+        ret = 0
+        i = 0
+        
+        while i < len(nums):
+            if nums[i] == val:
+                nums.remove(nums[i])
+                nums.append(None)
+            elif nums[i] == None:
+                i += 1
+            else:
+                i += 1
+                ret += 1
+                
+        return ret
\ No newline at end of file
diff --git a/1-100q/37.py b/1-100q/37.py
new file mode 100644
index 0000000..43a2d9d
--- /dev/null
+++ b/1-100q/37.py
@@ -0,0 +1,49 @@
+class Solution:
+    def solveSudoku(self, bo):
+        """
+        Do not return anything, modify board in-place instead.
+        """
+        find = self.find_empty(bo)
+        
+        if not find:
+            return True # solution found
+        
+        row, col = find
+        
+        for num in range(1, 10):
+            if self.is_valid(bo, row, col, num):
+                bo[row][col] = str(num)
+                
+                if self.solveSudoku(bo):
+                    return True
+                
+                bo[row][col] = "."
+        
+        return False
+    
+    def find_empty(self, bo):
+        for x in range(len(bo)):
+            for y in range(len(bo[x])):
+                if bo[x][y] == ".":
+                    return (x, y)
+    
+    def is_valid(self, bo, row, col, val):
+        val = str(val)
+        # check row and col
+        for x in range(len(bo)):
+            for y in range(len(bo[x])):
+                if bo[x][col] == val and x != row or bo[row][y] == val and y != col:
+                    return False
+        
+        # check 3x3 box
+        box_x = row // 3
+        box_y = col // 3
+        
+        for x in range(box_x * 3, box_x * 3 + 3):
+            for y in range(box_y * 3, box_y * 3 + 3):
+                if bo[x][y] == val and (x, y) != (row, col):
+                    return False
+        
+        return True
+    
+    
\ No newline at end of file
diff --git a/1-100q/43.py b/1-100q/43.py
new file mode 100644
index 0000000..6930653
--- /dev/null
+++ b/1-100q/43.py
@@ -0,0 +1,3 @@
+class Solution:
+    def multiply(self, num1: str, num2: str) -> str:
+        return str(int(num1) * int(num2))
\ No newline at end of file
diff --git a/1-100q/51.py b/1-100q/51.py
new file mode 100644
index 0000000..ce32c43
--- /dev/null
+++ b/1-100q/51.py
@@ -0,0 +1,46 @@
+class Solution:
+    def solveNQueens(self, n: int):
+        solutions = []
+        state = []
+        self.search(state, solutions, n)
+        return solutions
+    
+    def search(self, state, solutions, n):
+        if len(state) == n: # one more solution found
+            sol_str = self.to_string(state, n)
+            solutions.append(sol_str)
+            return
+        
+        for candidate in self.possible_candidates(state, n):
+            # recursion
+            state.append(candidate)
+            self.search(state, solutions, n)
+            state.pop()
+    
+    def possible_candidates(self, state, n):
+        if not state:
+            return range(n)
+        
+        position = len(state)
+        candidates = set(range(n))
+        
+        for row, col in enumerate(state):
+            # check row and col
+            candidates.discard(col)
+            # check diagonal
+            diagonal = position - row
+            
+            candidates.discard(col + diagonal)
+            candidates.discard(col - diagonal)
+        
+        return candidates
+    
+    def to_string(self, state, n):
+        # [1, 3, 0, 2] -> [".Q..", "...Q", "Q...", "..Q."]
+        result = []
+        
+        for i in state:
+            result.append("." * i + "Q" + "." * (n - i - 1))
+        
+        return result
+            
\ No newline at end of file
diff --git a/1-100q/52.py b/1-100q/52.py
new file mode 100644
index 0000000..2bf6fc6
--- /dev/null
+++ b/1-100q/52.py
@@ -0,0 +1,34 @@
+class Solution:
+    def totalNQueens(self, n: int) -> int:
+        solutions = []
+        state = []
+        self.search(solutions, state, n)
+        return len(solutions)
+    
+    def search(self, solutions, state, n):
+        if len(state) == n:
+            solutions.append(state)
+            return
+        
+        for candidate in self.possible_candidates(state, n):
+            state.append(candidate)
+            self.search(solutions, state, n)
+            state.pop()
+    
+    def possible_candidates(self, state, n):
+        if not state:
+            return range(n)
+        
+        position = len(state)
+        candidates = set(range(n))
+        
+        for row, col in enumerate(state):
+            # check row and col
+            candidates.discard(col)
+            
+            #check diaconal
+            dist = position - row
+            candidates.discard(col - dist)
+            candidates.discard(col + dist)
+        
+        return candidates
\ No newline at end of file
diff --git a/1-100q/7.py b/1-100q/7.py
new file mode 100644
index 0000000..26244c7
--- /dev/null
+++ b/1-100q/7.py
@@ -0,0 +1,11 @@
+class Solution:
+    def reverse(self, x: int) -> int:
+        revx = int(str(x)[::-1].replace("-", ""))
+        
+        if x < 0:
+            revx *= -1
+            
+        if revx >= 2**31-1 or revx <= -2**31:
+            return 0
+        
+        return revx
\ No newline at end of file
diff --git a/1-100q/88.py b/1-100q/88.py
new file mode 100644
index 0000000..4828ce6
--- /dev/null
+++ b/1-100q/88.py
@@ -0,0 +1,17 @@
+class Solution:
+    def merge(self, nums1, m, nums2, n):
+        for i in range(len(nums1)):
+            if i >= m:
+                nums1.pop()
+        
+        i = 0
+        
+        while nums2 and i < len(nums1):
+            if nums2[0] <= nums1[i]:
+                nums1.insert(i, nums2[0])
+                nums2.pop(0)
+            else:
+                i += 1
+            
+        if nums2:
+            nums1.extend(nums2)