Given a binary tree, return the preorder traversal of its nodes’ values.
Example:
Input: [1,null,2,3]
1
2
/
3
Output: [1,2,3]
Follow up: Recursive solution is trivial, could you do it iteratively?
递归:
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> pre = new LinkedList<Integer>();
prehelper(root, pre);
return pre;
}
private void prehelper(TreeNode root, List<Integer> pre){
if(root == null) return;
pre.add(root.val);
prehelper(root.left, pre);
prehelper(root.right, pre);
}
迭代:
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> pre = new LinkedList<Integer>();
if(root == null) return pre;
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while(!stack.isEmpty()){
TreeNode node = stack.pop();
pre.add(node.val);
if(node.right != null) stack.push(node.right);
if(node.left != null) stack.push(node.left);
}
return pre;
}