diff --git a/src/main/java/com/cool/ling_cha_mount/binary_search/Num981.java b/src/main/java/com/cool/ling_cha_mount/binary_search/Num981.java
new file mode 100644
index 0000000..cc3fcef
--- /dev/null
+++ b/src/main/java/com/cool/ling_cha_mount/binary_search/Num981.java
@@ -0,0 +1,53 @@
+package com.cool.ling_cha_mount.binary_search;
+
+import java.util.*;
+import java.util.stream.Collectors;
+
+public class Num981 {
+
+
+}
+
+class TimeMap {
+
+    private final Map<String, List<Integer>> timeMap;
+    private final Map<String, List<String>> valueMap;
+
+    public TimeMap() {
+        timeMap = new HashMap<>();
+        valueMap = new HashMap<>();
+    }
+
+    public void set(String key, String value, int timestamp) {
+        timeMap.computeIfAbsent(key, e -> new ArrayList<>()).add(timestamp);
+        valueMap.computeIfAbsent(key, e -> new ArrayList<>()).add(value);
+    }
+
+    public String get(String key, int timestamp) {
+        if (!timeMap.containsKey(key)) {
+            return "";
+        }
+        List<Integer> timeList = timeMap.get(key);
+        List<String> valueList = valueMap.get(key);
+        int index = binarySearch(timeList, timestamp + 1) - 1;
+        if (index < 0) {
+            return "";
+        }
+        return valueList.get(index);
+
+    }
+
+    private int binarySearch(List<Integer> list, int target) {
+        int left = -1;
+        int right = list.size();
+        while (left + 1 < right) {
+            int mid = (left + right) >>> 1;
+            if (list.get(mid) >= target) {
+                right = mid;
+            } else {
+                left = mid;
+            }
+        }
+        return right;
+    }
+}