English
145. Binary Tree Postorder Traversal 
Problem Statement:
Given the root of a binary tree, return the postorder traversal of its nodes' values.
Example 1:
Input: root = [1,null,2,3] Output: [3,2,1]
Example 2:
Input: root = []
Output: []
Example 3:
Input: root = [1]
Output: [1]
Constraints:
- The number of the nodes in the tree is in the range [0, 100].
- -100 <= Node.val <= 100
Follow up: Recursive solution is trivial, could you do it iteratively?
Solution:
java
private LinkedList<Integer> list = new LinkedList<>();
public List<Integer> postorderTraversal(TreeNode root) {
if (root == null)
return list;
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while (!stack.isEmpty()) {
TreeNode current = stack.pop();
list.addFirst(current.val);
if (current.left != null)
stack.push(current.left);
if (current.right != null)
stack.push(current.right);
}
return list;
}
public List<Integer> postorderTraversalRecursive(TreeNode root) {
if (root == null)
return list;
postorderTraversalRecursive(root.left);
postorderTraversalRecursive(root.right);
list.add(root.val);
return list;
}
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