144. Binary Tree Preorder Traversal

1. Question

Given the root of a binary tree, return the preorder traversal of its nodes' values.

2. Examples

Example 1:

Input: root = [1,null,2,3]
Output: [1,2,3]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Example 4:

Input: root = [1,2]
Output: [1,2]

Example 5:

Input: root = [1,null,2]
Output: [1,2]

3. Constraints

• The number of 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?

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/binary-tree-preorder-traversal 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

class Solution {
  List<Integer> list = new ArrayList<>();

  public List<Integer> preorderTraversal(TreeNode root) {
    if(root != null){
      search(root);
    }
    return list;
  }

  public void search(TreeNode root) {
    list.add(root.val);
    if (root.left != null) {
      search(root.left);
    }
    if (root.right != null) {
      search(root.right);
    }
  }
}