feat(0504_base-7): add py3 soln

0504_base-7/README.md

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+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:**
+
+*   `-107 <= num <= 107`

0504_base-7/python3/solution.py

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+class Solution:
+    def convertToBase7(self, num: int) -> str:
+        sign = '-' if num < 0 else ''
+        num = abs(num)
+        b7 = ''
+        
+        while num > 0:
+            b7 = str(num % 7) + b7
+            num //= 7
+        
+        return '0' if b7 == '' else sign + b7