[N-0] refactor 121

Author: fishercoder1534

README.md

@@ -522,7 +522,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |124|[Binary Tree Maximum Path Sum](https://leetcode.com/problems/binary-tree-maximum-path-sum/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_124.java)| O(n)|O(h) | Hard | Tree, DFS
 |123|[Best Time to Buy and Sell Stock III](https://leetcode.com/problems/best-time-to-buy-and-sell-stock-iii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_123.java)| O(?)|O(?) | Hard |
 |122|[Best Time to Buy and Sell Stock II](https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_122.java)| O(n)|O(1) | Medium | Greedy
-|121|[Best Time to Buy and Sell Stock](https://leetcode.com/problems/best-time-to-buy-and-sell-stock/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_121.java)| O(n)|O(1) | Easy| DP
+|121|[Best Time to Buy and Sell Stock](https://leetcode.com/problems/best-time-to-buy-and-sell-stock/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_121.java)| O(n)|O(1) | Easy|
 |120|[Triangle](https://leetcode.com/problems/triangle/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_120.java)| O(m*n)|O(n) | Medium| DP
 |119|[Pascal's Triangle II](https://leetcode.com/problems/pascals-triangle-ii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_119.java)| O(n^2)|O(k) | Easy| 
 |118|[Pascal's Triangle](https://leetcode.com/problems/pascals-triangle/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_118.java)| O(n^2)|O(1) | Easy| 

src/main/java/com/fishercoder/solutions/_121.java

@@ -30,25 +30,18 @@ public class _121 {
      * before you buy it.
      */
     public int maxProfit(int[] prices) {
-        if (prices == null || prices.length < 2) {
+        if (prices == null || prices.length == 0) {
             return 0;
         }
-        int minBuy = prices[0];
-        int maxSell = prices[1];
-        int maxProfit = (maxSell - minBuy) > 0 ? (maxSell - minBuy) : 0;
+        int buy = prices[0];
+        int maxProfit = 0;
         for (int i = 1; i < prices.length; i++) {
-            minBuy = Math.min(minBuy, prices[i]);
-            maxProfit = Math.max(maxProfit, prices[i] - minBuy);
+            if (prices[i] < buy) {
+                buy = prices[i];
+            } else {
+                maxProfit = Math.max(maxProfit, prices[i] - buy);
+            }
         }
         return maxProfit;
     }
-
-    public static void main(String... strings) {
-//        int[] prices = new int[]{7,1,5,3,6,4};
-//        int[] prices = new int[]{7,6,4,3,1};
-//        int[] prices = new int[]{2,4,1};
-        int[] prices = new int[]{1, 2};
-        _121 test = new _121();
-        System.out.println(test.maxProfit(prices));
-    }
 }

src/test/java/com/fishercoder/_121Test.java

@@ -0,0 +1,42 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._121;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static junit.framework.Assert.assertEquals;
+
+public class _121Test {
+    private static _121 test;
+    private static int[] prices;
+
+    @BeforeClass
+    public static void setup() {
+        test = new _121();
+    }
+
+    @Test
+    public void test1() {
+        prices = new int[]{7, 1, 5, 3, 6, 4};
+        assertEquals(5, test.maxProfit(prices));
+    }
+
+    @Test
+    public void test2() {
+        prices = new int[]{7, 6, 4, 3, 1};
+        assertEquals(0, test.maxProfit(prices));
+    }
+
+    @Test
+    public void test3() {
+        prices = new int[]{2, 4, 1};
+        assertEquals(2, test.maxProfit(prices));
+    }
+
+    @Test
+    public void test4() {
+        prices = new int[]{1, 2};
+        assertEquals(1, test.maxProfit(prices));
+    }
+
+}