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< p > Given the < code > root< / code > of an n-ary tree, return < em > the postorder traversal of its nodes' values< / em > .< / p >
< p > Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)< / p >
< p > < / p >
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< p > < strong class = "example" > Example 1:< / strong > < / p >
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< img src = "https://assets.leetcode.com/uploads/2018/10/12/narytreeexample.png" style = "width: 100%; max-width: 300px;" / >
< pre >
< strong > Input:< / strong > root = [1,null,3,2,4,null,5,6]
< strong > Output:< / strong > [5,6,3,2,4,1]
< / pre >
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< p > < strong class = "example" > Example 2:< / strong > < / p >
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< img alt = "" src = "https://assets.leetcode.com/uploads/2019/11/08/sample_4_964.png" style = "width: 296px; height: 241px;" / >
< pre >
< strong > Input:< / strong > root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
< strong > Output:< / strong > [2,6,14,11,7,3,12,8,4,13,9,10,5,1]
< / pre >
< p > < / p >
< p > < strong > Constraints:< / strong > < / p >
< ul >
< li > The number of nodes in the tree is in the range < code > [0, 10< sup > 4< / sup > ]< / code > .< / li >
< li > < code > 0 < = Node.val < = 10< sup > 4< / sup > < / code > < / li >
< li > The height of the n-ary tree is less than or equal to < code > 1000< / code > .< / li >
< / ul >
< p > < / p >
< p > < strong > Follow up:< / strong > Recursive solution is trivial, could you do it iteratively?< / p >
