diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index ba63524..0c44635 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -96,4 +96,5 @@ mod p26_remove_duplicates_from_sorted_array;
 mod p27_remove_element;
 mod p80_remove_duplicates_from_sorted_array_ii;
 mod p169_majority_element;
-mod p189_rotate_array;
\ No newline at end of file
+mod p189_rotate_array;
+mod p121_best_time_to_buy_and_sell_stock;
\ No newline at end of file
diff --git a/src/problem/p121_best_time_to_buy_and_sell_stock.rs b/src/problem/p121_best_time_to_buy_and_sell_stock.rs
new file mode 100644
index 0000000..f50ad72
--- /dev/null
+++ b/src/problem/p121_best_time_to_buy_and_sell_stock.rs
@@ -0,0 +1,38 @@
+/**
+ * [121] Best Time to Buy and Sell Stock
+ */
+pub struct Solution {}
+
+
+// submission codes start here
+
+impl Solution {
+    pub fn max_profit(prices: Vec<i32>) -> i32 {
+        let mut result = 0;
+        let mut min_num = prices[0];
+
+        for i in 1..prices.len() {
+            if prices[i] > prices[i - 1] {
+                result = result.max(prices[i] - min_num);
+            } else {
+                if prices[i] < min_num {
+                    min_num = prices[i];
+                }
+            }
+        }
+
+        result
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_121() {
+        assert_eq!(1, Solution::max_profit(vec![1,2]));
+    }
+}