Question Definition
Given a binary tree, return the preorder traversal of its nodes’ values.
For example:
Given binary tree [1,null,2,3],
1
\
2
/
3
return [1,2,3].
Note: Recursive solution is trivial, could you do it iteratively?
Java Solution
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result = new LinkedList<>();
if(root == null) return result;
Deque<TreeNode> deque = new ArrayDeque<>();
deque.push(root);
while(!deque.isEmpty()){
TreeNode temp = deque.pop();
result.add(temp.val);
if(temp.right != null) deque.push(temp.right);
if(temp.left != null) deque.push(temp.left);
}
return result;
}
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