feat(54-spiral-matrix): add py3 solution

0054_spiral-matrix/README.md

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+Given an `m x n` `matrix`, return _all elements of the_ `matrix` _in spiral order_.
+
+Example 1:
+
+![](https://assets.leetcode.com/uploads/2020/11/13/spiral1.jpg)
+    
+    Input: matrix = [[1,2,3],[4,5,6],[7,8,9]] Output: [1,2,3,6,9,8,7,4,5] 
+
+Example 2:
+
+![](https://assets.leetcode.com/uploads/2020/11/13/spiral.jpg)
+    
+    Input: matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]] Output: [1,2,3,4,8,12,11,10,9,5,6,7] 
+
+Constraints:
+
+* `m == matrix.length`
+* `n == matrix[i].length`
+* `1 <= m, n <= 10`
+* `-100 <= matrix[i][j] <= 100`

0054_spiral-matrix/python3/solution.py

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+class Solution:
+    def spiralOrder(self, matrix: List[List[int]]) -> List[int]:
+        row_begin_loc, row_end_loc = 0, len(matrix) - 1
+        col_begin_loc, col_end_loc = 0, len(matrix[0]) - 1
+        result = []
+        
+        while row_begin_loc <= row_end_loc and col_begin_loc <= col_end_loc:
+            # Top Side
+            i, j = row_begin_loc, col_begin_loc
+            while j <= col_end_loc:
+                result.append(matrix[i][j])
+                j += 1
+
+            # Right Side
+            i += 1
+            j -= 1
+            while i <= row_end_loc:
+                result.append(matrix[i][j])
+                i += 1
+
+            # Bottom Side
+            i -= 1
+            j -= 1
+            while j >= col_begin_loc and i > row_begin_loc:
+                result.append(matrix[i][j])
+                j -= 1
+            
+            # Left Side
+            i -= 1
+            j += 1
+            while i > row_begin_loc and j < col_end_loc:
+                result.append(matrix[i][j])
+                i -= 1
+            
+            row_begin_loc += 1
+            row_end_loc -= 1
+            col_begin_loc += 1
+            col_end_loc -= 1
+        
+        return result
+