Added solutions

Author: adityaramkumar

557.py

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+# Runtime: 28 ms, faster than 76.71% of Python online submissions for Reverse Words in a String III.
+# Difficulty: Easy
+
+class Solution(object):
+    def reverseWords(self, s):
+        """
+        :type s: str
+        :rtype: str
+        """
+        return ' '.join(map(lambda word: word[::-1], s.split(' ')))

590.py

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+# Runtime: 112 ms, faster than 98.45% of Python online submissions for N-ary Tree Postorder Traversal.
+# Difficulty: Easy
+
+class Solution(object):
+    def postorder(self, root):
+        """
+        :type root: Node
+        :rtype: List[int]
+        """
+        if not root:
+            return []
+        p_list = list()
+        for child in root.children:
+            p_list.extend(self.postorder(child))
+        p_list.append(root.val)
+        return p_list

627.sql

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+# Runtime: 330 ms, faster than 35.42% of MySQL online submissions for Swap Salary.
+# Difficulty: Easy
+
+UPDATE SALARY
+SET sex = IF(sex = 'f', 'm', 'f');