add: leetcode 0504 solution

leetcode/0504.Base-7/504.Base 7.go

@@ -0,0 +1,28 @@
+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
+}