590. N-ary Tree Postorder Traversal
Description
Given the root
of an n-ary tree, return the postorder traversal of its nodes' values.
Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)
Example 1:
Input: root = [1,null,3,2,4,null,5,6] Output: [5,6,3,2,4,1]
Example 2:
Input: 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] Output: [2,6,14,11,7,3,12,8,4,13,9,10,5,1]
Constraints:
- The number of nodes in the tree is in the range
[0, 104]
. 0 <= Node.val <= 104
- The height of the n-ary tree is less than or equal to
1000
.
Follow up: Recursive solution is trivial, could you do it iteratively?
Solutions
Solution 1: Recursion
We can recursively traverse the entire tree. For each node, we first recursively call the function for each of the node's children, then add the node's value to the answer.
The time complexity is $O(n)$, and the space complexity is $O(n)$. Here, $n$ is the number of nodes.
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Solution 2: Iteration (Stack Implementation)
We can also solve this problem iteratively.
We use a stack to help us get the post-order traversal. We first push the root node into the stack. Since the post-order traversal is left subtree, right subtree, root, and the characteristic of the stack is first in last out, we first add the node's value to the answer, then push each of the node's children into the stack in the order from left to right. This way, we can get the traversal result of root, right subtree, left subtree. Finally, we reverse the answer to get the post-order traversal result.
The time complexity is $O(n)$, and the space complexity is $O(n)$. Here, $n$ is the number of nodes.
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