diff --git "a/leetcode3/\354\227\274\355\230\234\354\240\225/3803. Count Residue Prefixes.java" "b/leetcode3/\354\227\274\355\230\234\354\240\225/3803. Count Residue Prefixes.java"
new file mode 100644
index 00000000..7e4aebbd
--- /dev/null
+++ "b/leetcode3/\354\227\274\355\230\234\354\240\225/3803. Count Residue Prefixes.java"	
@@ -0,0 +1,21 @@
+// 문자열의 0번째부터 set에 있는지 확인
+// 없으면 넣어주고 distinct_cnt + 1
+// O(n)
+
+class Solution {
+    public int residuePrefixes(String s) {
+        Set<Character> set = new HashSet<>();
+        int distinct_cnt = 0;
+        int result = 0;
+        for (int i = 0; i<s.length(); i++) {
+            char c = s.charAt(i);
+            if (!set.contains(c)) { // 중복이 아니라면
+                distinct_cnt++;
+                set.add(c);
+            }
+            int modulo = (i + 1) % 3;
+            if (modulo == distinct_cnt) result++;
+        }
+        return result;
+    }
+}