144 Binary Tree Preorder Traversal
- Binary Tree Preorder Traversal 遞迴寫法 終止條件，若為葉子，把結果返回
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class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
if(root == null) return res;
res.add(root.val);
res.addAll(preorderTraversal(root.left));
res.addAll(preorderTraversal(root.right));
return res;
}
}