feat: add the solution of Base 7(504) with kotlin.

Author: relish-wang

README.md

@@ -60,6 +60,7 @@
 | [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] |              | [✅][122-kotlin] |
 | [226][226-question] | [Invert Binary Tree][226-tips]                               | [✅][226-java] | [✅][226-js] | [✅][226-kotlin] |
+| [504][504-question] | [Base 7][504-tips]                                           |                |              | [✅][504-kotlin] |
 | [543][543-question] | [Diameter of Binary Tree][543-tips]                          | [✅][543-java] |              | [✅][543-kotlin] |
 | [561][561-question] | [Array Partition I][561-tips]                                | [✅][561-java] | [✅][561-js] |                  |
 | [643][643-question] | [Maximum Average Subarray I][643-tips]                       | [✅][643-java] | [✅][643-js] |                  |
@@ -212,6 +213,7 @@
 [121-question]: https://leetcode.com/problems/best-time-to-buy-and-sell-stock/description/
 [122-question]: https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/description/
 [226-question]: https://leetcode.com/problems/invert-binary-tree/
+[504-question]: https://leetcode.com/problems/base-7/description/
 [543-question]: https://leetcode.com/problems/diameter-of-binary-tree/
 [554-question]: https://leetcode.com/problems/brick-wall/description/
 [561-question]: https://leetcode.com/problems/array-partition-i/
@@ -522,5 +524,6 @@
 [121-kotlin]: ./src/_121/kotlin/Solution.kt
 [122-kotlin]: ./src/_122/kotlin/Solution.kt
 [226-kotlin]: ./src/_226/kotlin/Solution.kt
+[504-kotlin]: ./src/_504/kotlin/Solution.kt
 [543-kotlin]: ./src/_543/kotlin/Solution.kt
 [771-kotlin]: ./src/_771/kotlin/Solution.kt

src/_504/kotlin/Solution.kt

@@ -0,0 +1,28 @@
+package _504.kotlin
+
+import java.util.*
+
+/**
+ * @author relish
+ * @since 2018/10/19
+ */
+class Solution {
+    fun convertToBase7(num: Int): String {
+        if (num == 0) return "0"
+        val isPositive = num >= 0
+        var n = Math.abs(num)
+        val sb = StringBuilder()
+        while (n > 0) {
+            sb.append(n % 7)
+            n /= 7
+        }
+        sb.append(if (isPositive) "" else "-")
+        return sb.reverse().toString()
+    }
+}
+
+fun main(args: Array<String>) {
+    val scanner = Scanner(System.`in`)
+    val input = scanner.nextInt()
+    println(_504.kotlin.Solution().convertToBase7(input))
+}

tips/504/README.md

@@ -0,0 +1,52 @@
+[Base 7][title]
+
+## Description
+
+Given an integer, return its base 7 string representation.
+
+
+**Example 1:**
+
+```
+Input: 100
+Output: "202"
+```
+
+**Example 2:**
+
+```Input: -7
+   Output: "-10"
+```
+
+**Note:**
+The input will be in range of [-1e7, 1e7].
+
+**Tags:** [no tags]
+
+
+## 思路 1
+判断一下0的情况和负数的符号
+kotlin(192ms/100.00%):
+```kotlin
+class Solution {
+    fun convertToBase7(num: Int): String {
+        if (num == 0) return "0"
+        val isPositive = num >= 0
+        var n = Math.abs(num)
+        val sb = StringBuilder()
+        while (n > 0) {
+            sb.append(n % 7)
+            n /= 7
+        }
+        sb.append(if (isPositive) "" else "-")
+        return sb.reverse().toString()
+    }
+}
+```
+
+## 结语
+   
+如果你同我们一样热爱数据结构、算法、LeetCode，可以关注我们 GitHub 上的 LeetCode 题解：[LeetCode-Solution][ls]
+
+[title]: https://leetcode.com/problems/base-7/description/
+[ls]: https://github.com/RichCodersAndMe/LeetCode-Solution