feat: add the solution of Best Time to Buy and Sell Stock(121) with kotlin.

Author: relish-wang

README.md

@@ -57,7 +57,7 @@
 | [112][112-question] | [Path Sum][112-tips]                                         | [✅][112-java] |              | [✅][112-kotlin] |
 | [118][118-question] | [Pascal's Triangle][118-tips]                                | [✅][118-java] |              | [✅][118-kotlin] |
 | [119][119-question] | [Pascal's Triangle II][119-tips]                             | [✅][119-java] |              | [✅][119-kotlin] |
-| [121][121-question] | [Best Time to Buy and Sell Stock][121-tips]                  | [✅][121-java] |              |                  |
+| [121][121-question] | [Best Time to Buy and Sell Stock][121-tips]                  | [✅][121-java] |              | [✅][121-kotlin] |
 | [122][122-question] | [Best Time to Buy and Sell Stock II][122-tips]               | [✅][122-java] |              |                  |
 | [226][226-question] | [Invert Binary Tree][226-tips]                               | [✅][226-java] | [✅][226-js] |                  |
 | [543][543-question] | [Diameter of Binary Tree][543-tips]                          | [✅][543-java] |              |                  |
@@ -516,4 +516,5 @@
 [112-kotlin]: ./src/_112/kotlin/Solution.kt
 [118-kotlin]: ./src/_118/kotlin/Solution.kt
 [119-kotlin]: ./src/_119/kotlin/Solution.kt
+[121-kotlin]: ./src/_121/kotlin/Solution.kt
 [771-kotlin]: ./src/_771/kotlin/Solution.kt

src/_121/kotlin/Solution.kt

@@ -0,0 +1,23 @@
+package _121.kotlin
+
+/**
+ * @author relish
+ * @since 2018/05/07
+ */
+class Solution {
+    fun maxProfit(prices: IntArray): Int {
+        if (prices.isEmpty()) return 0
+        var ret = 0
+        var min = prices[0]
+        for (i in 1 until prices.size) {
+            min = Math.min(min, prices[i])
+            ret = Math.max(prices[i] - min, ret)
+        }
+        return ret
+    }
+}
+
+fun main(args: Array<String>) {
+    println(Solution().maxProfit(intArrayOf(7, 1, 5, 3, 6, 4)))
+    println(Solution().maxProfit(intArrayOf(7, 6, 4, 3, 1)))
+}

tips/121/README.md

@@ -31,6 +31,7 @@ In this case, no transaction is done, i.e. max profit = 0.
 
 题意是给出一个数组代表每天的股票金额，让你在最多买卖一次的情况下算出最大的收益额，最简单的就是模拟即可，每次记录当前值减去最小值的差值，与上一次的进行比较然后更新最大值即可。
 
+Java:
 ```java
 class Solution {
     public int maxProfit(int[] prices) {
@@ -45,6 +46,21 @@ class Solution {
 }
 ```
 
+kotlin(204ms/100.00%):
+```kotlin
+class Solution {
+    fun maxProfit(prices: IntArray): Int {
+        if (prices.isEmpty()) return 0
+        var ret = 0
+        var min = prices[0]
+        for (i in 1 until prices.size) {
+            min = Math.min(min, prices[i])
+            ret = Math.max(prices[i] - min, ret)
+        }
+        return ret
+    }
+}
+```
 
 ## 结语