diff --git "a/LiiNi-coder/202508/21 PGM \352\264\221\352\263\240 \354\202\275\354\236\205.md" "b/LiiNi-coder/202508/21 PGM \352\264\221\352\263\240 \354\202\275\354\236\205.md"
new file mode 100644
index 00000000..9d68cae5
--- /dev/null
+++ "b/LiiNi-coder/202508/21 PGM \352\264\221\352\263\240 \354\202\275\354\236\205.md"	
@@ -0,0 +1,115 @@
+```java
+import java.util.*;
+
+class Solution {
+    static class TimeInfo{
+        int startCount; //time에 시작하는 log개수
+        int endCount;//time에 끝나는 log개수
+        long prefix; //time까지의 가중치 누적합
+        
+        TimeInfo(int sc, int ec, long p){
+            this.startCount = sc;
+            this.endCount = ec;
+            this.prefix = p;
+        }
+        @Override
+        public String toString(){
+            return String.format("sc: %d, ec: %d prefix:%d", this.startCount, this.endCount, this.prefix);
+        }
+    }
+    
+    private static TreeMap<Integer, TimeInfo> timeInfos;
+    public String solution(String play_time, String adv_time, String[] logs) {
+        timeInfos = new TreeMap<Integer, TimeInfo>();
+        timeInfos.put(0, new TimeInfo(0, 0, 0));
+
+        for(String log : logs){
+            String[] temp = log.split("-");
+            int startTime = convertToSeconds(temp[0]);
+            int endTime = convertToSeconds(temp[1]);
+            
+            timeInfos.computeIfAbsent(startTime, k -> new TimeInfo(0, 0, 0)).startCount++;
+            timeInfos.computeIfAbsent(endTime, k -> new TimeInfo(0, 0, 0)).endCount++;
+        }
+        
+        //prefix 누적합 계산
+        long currentLogs = 0;
+        long cumulativeSum = 0;
+        Integer prevTime = null;
+        for(Map.Entry<Integer, TimeInfo> entry : timeInfos.entrySet()){
+            int time = entry.getKey();
+            TimeInfo info = entry.getValue();
+            
+            if(prevTime != null){
+                cumulativeSum += currentLogs * (time - prevTime);
+            }
+            info.prefix = cumulativeSum;
+            currentLogs += info.startCount - info.endCount;
+            prevTime = time;
+        }
+
+        int advSeconds = convertToSeconds(adv_time);
+        int playSeconds = convertToSeconds(play_time);
+        long maxViewTime = 0;
+        int bestStartTime = 0;
+        for(int startTime = 0; startTime <= playSeconds - advSeconds; startTime++){
+            int endTime = startTime + advSeconds;
+            long viewTime = getViewTime(startTime, endTime);
+            
+            if(viewTime > maxViewTime){
+                maxViewTime = viewTime;
+                bestStartTime = startTime;
+            }
+        }
+        
+        return convertToHHMMSS(bestStartTime);
+    }
+    
+    //startTime부터 endTime까지의 총 재생시간 계산
+    private long getViewTime(int startTime, int endTime){
+        long endPrefix = getPrefixSum(endTime);
+        long startPrefix = getPrefixSum(startTime);
+        return endPrefix - startPrefix;
+    }
+    
+    //time 시점까지의 누적 재생시간
+    private long getPrefixSum(int time){
+        Map.Entry<Integer, TimeInfo> floorEntry = timeInfos.floorEntry(time);
+        if(floorEntry == null) return 0;
+        
+        int floorTime = floorEntry.getKey();
+        TimeInfo floorInfo = floorEntry.getValue();
+        long prefix = floorInfo.prefix;
+        
+        //구간 내 재생시간 추가 계산
+        if(time > floorTime){
+            long currentLogs = 0;
+            // floorTime 이후의 로그 수 계산
+            for(Map.Entry<Integer, TimeInfo> entry : timeInfos.headMap(floorTime, true).entrySet()){
+                TimeInfo info = entry.getValue();
+                currentLogs += info.startCount - info.endCount;
+            }
+            prefix += currentLogs * (time - floorTime);
+        }
+        return prefix;
+    }
+    
+    private int convertToSeconds(String hms){
+        String[] tokens = hms.split(":");
+        int result = 0;
+        for(int i = 2, gop = 1; i>=0; i--, gop*=60){
+            result += Integer.parseInt(tokens[i])*gop;
+        }
+        return result;
+    }
+    
+    private String convertToHHMMSS(int seconds){
+        int h = seconds / 3600;
+        seconds %= 3600;
+        int m = seconds / 60;
+        int s = seconds % 60;
+        return String.format("%02d:%02d:%02d", h, m, s);
+    }
+}
+
+```