diff --git "a/week1/4. \353\254\270\354\236\220\354\227\264 \353\202\264 \353\247\210\354\235\214\353\214\200\353\241\234 \354\240\225\353\240\254\355\225\230\352\270\260/Solution.java" "b/week1/4. \353\254\270\354\236\220\354\227\264 \353\202\264 \353\247\210\354\235\214\353\214\200\353\241\234 \354\240\225\353\240\254\355\225\230\352\270\260/Solution.java"
index a37b2fe..c18e907 100644
--- "a/week1/4. \353\254\270\354\236\220\354\227\264 \353\202\264 \353\247\210\354\235\214\353\214\200\353\241\234 \354\240\225\353\240\254\355\225\230\352\270\260/Solution.java"	
+++ "b/week1/4. \353\254\270\354\236\220\354\227\264 \353\202\264 \353\247\210\354\235\214\353\214\200\353\241\234 \354\240\225\353\240\254\355\225\230\352\270\260/Solution.java"	
@@ -1,11 +1,117 @@
+import java.util.Arrays;
+
 /*
     4. 문자 내마음대로 정렬하기
     https://programmers.co.kr/learn/courses/30/lessons/12915
 */
-class Solution {
+class Solution4 {
+
+    public static void main(String[] args) {
+
+        Solution4 sol = new Solution4();
+
+        String[] strArray = { "abce", "abcd", "cdx" };
+        int nthChar = 2;
+
+        sol.solution(strArray, nthChar);
+
+    }
 
     public String[] solution(String[] strings, int n) {
-        return null;
+
+        String[] tempStringArray = new String[strings.length];
+
+        mergeSort(strings, 0, strings.length - 1, n, tempStringArray);
+
+        System.out.println(Arrays.toString(strings));
+
+        return strings;
+    }
+
+    public void mergeSort(String[] strArray, int start, int end, int n, String[] tempStringArray) {
+
+        if (start < end) {
+
+            int medium = (start + end) / 2;
+
+            mergeSort(strArray, start, medium, n, tempStringArray);
+            mergeSort(strArray, medium + 1, end, n, tempStringArray);
+
+            int i = start;
+            int j = medium + 1;
+            int k = start;
+            while (i <= medium && j <= end) {
+                if (strArray[i].charAt(n) < strArray[j].charAt(n)) {
+
+                    tempStringArray[k++] = strArray[i++];
+
+                } else if (strArray[i].charAt(n) == strArray[j].charAt(n)) {
+
+                    int tempIndexDownCount = 1;
+                    int tempIndexUpperCount = 1;
+
+                    boolean isFinished = false;
+
+                    while ((n - tempIndexDownCount) != 0) {
+                        if (strArray[i].charAt(n - tempIndexDownCount) < strArray[j].charAt(n - tempIndexDownCount)) {
+
+                            tempStringArray[k++] = strArray[i++];
+                            isFinished = true;
+                            break;
+                        } else if (strArray[i].charAt(n - tempIndexDownCount) == strArray[j]
+                                .charAt(n - tempIndexDownCount)) {
+
+                        } else {
+                            tempStringArray[k++] = strArray[j++];
+                        }
+
+                        ++tempIndexDownCount;
+
+                    }
+
+                    while (!isFinished && (n + tempIndexUpperCount) != strArray[i].length() - 1) {
+                        if (strArray[i].charAt(n + tempIndexUpperCount) < strArray[j].charAt(n + tempIndexUpperCount)) {
+
+                            tempStringArray[k++] = strArray[i++];
+                            isFinished = true;
+                            break;
+                        } else if (strArray[i].charAt(n + tempIndexUpperCount) == strArray[j]
+                                .charAt(n + tempIndexUpperCount)) {
+
+                        } else {
+                            tempStringArray[k++] = strArray[j++];
+                        }
+
+                        ++tempIndexUpperCount;
+                    }
+
+                    if (!isFinished && strArray[i].length() > strArray[j].length()) {
+                        tempStringArray[k++] = strArray[j++];
+                    } else if (!isFinished && strArray[i].length() < strArray[j].length()) {
+                        tempStringArray[k++] = strArray[i++];
+                    } else {
+                        tempStringArray[k++] = strArray[j++];
+                    }
+
+                } else {
+
+                    tempStringArray[k++] = strArray[j++];
+
+                }
+            }
+
+            while (i <= medium) {
+                tempStringArray[k++] = strArray[i++];
+            }
+            while (j <= end) {
+                tempStringArray[k++] = strArray[j++];
+            }
+            for (int c = start; c <= end; c++) {
+                strArray[c] = tempStringArray[c];
+            }
+
+        }
+
     }
 
 }