add: leetcode 0504 readme

leetcode/0504.Base-7/README.md

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+# [504. Base 7](https://leetcode-cn.com/problems/base-7/)
+
+## 题目
+
+Given an integer num, return a string of its base 7 representation.
+
+**Example 1:**
+
+    Input: num = 100
+    Output: "202"
+
+**Example 2:**
+
+    Input: num = -7
+    Output: "-10"
+
+**Constraints:**
+
+- -10000000 <= num <= 10000000
+
+## 题目大意
+
+给定一个整数 num，将其转化为 7 进制，并以字符串形式输出。
+
+## 解题思路
+
+    num反复除以7，然后倒排余数
+
+# 代码
+
+```go
+package leetcode
+
+import "strconv"
+
+func convertToBase7(num int) string {
+	if num == 0 {
+		return "0"
+	}
+	negative := false
+	if num < 0 {
+		negative = true
+		num = -num
+	}
+	var ans string
+	var nums []int
+	for num != 0 {
+		remainder := num % 7
+		nums = append(nums, remainder)
+		num = num / 7
+	}
+	if negative {
+		ans += "-"
+	}
+	for i := len(nums) - 1; i >= 0; i-- {
+		ans += strconv.Itoa(nums[i])
+	}
+	return ans
+}
+```