diff --git a/src/main/java/com/cool/hot100/greedy/Num121.java b/src/main/java/com/cool/hot100/greedy/Num121.java
new file mode 100644
index 0000000..1855f0d
--- /dev/null
+++ b/src/main/java/com/cool/hot100/greedy/Num121.java
@@ -0,0 +1,32 @@
+package com.cool.hot100.greedy;
+
+import org.junit.Test;
+
+/**
+ * Created with IntelliJ IDEA.
+ *
+ * @Author: Cool
+ * @Date: 2024/09/21/23:06
+ * DayNumber 3
+ * Hard 1
+ * Level 3
+ */
+public class Num121 {
+
+    public int maxProfit(int[] prices) {
+        int minPrice=prices[0];
+        int maxProfit=0;
+        for (int i = 1; i < prices.length; i++) {
+            if(prices[i]<minPrice){
+                minPrice=prices[i];
+            }else if(prices[i]-minPrice>maxProfit){
+                maxProfit=prices[i]-minPrice;
+            }
+        }
+        return maxProfit;
+    }
+    @Test
+    public void test(){
+        maxProfit(new int[]{7,1,5,3,6,4});
+    }
+}