Adding readme file

Author: bugsweeper

145-BinaryTreePostorderTraversal/README.md

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+Given the root of a binary tree, return the postorder traversal of its nodes' values.
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+Example 1:
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+Input: root = [1,null,2,3]
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+Output: [3,2,1]
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+Explanation:
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+Example 2:
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+Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]
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+Output: [4,6,7,5,2,9,8,3,1]
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+Explanation:
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+Example 3:
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+Input: root = []
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+Output: []
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+Example 4:
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+Input: root = [1]
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+Output: [1]
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+ 
+Constraints:
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+	The number of the nodes in the tree is in the range [0, 100].
+	-100 <= Node.val <= 100
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+ 
+Follow up: Recursive solution is trivial, could you do it iteratively?