java: add «121. Best Time to Buy and Sell Stock»

URL:	https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
Signed-off-by: Matej Focko <me@mfocko.xyz>

java/best-time-to-buy-and-sell-stock.java

@@ -0,0 +1,11 @@
+class Solution {
+  public int maxProfit(int[] prices) {
+    int buy = prices[0], profit = 0;
+    for (var price : prices) {
+      buy = Math.min(buy, price);
+      profit = Math.max(profit, price - buy);
+    }
+
+    return profit;
+  }
+}