Solution1.java

/// Source : https://leetcode.com/problems/binary-tree-preorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2017-11-17
import java.util.ArrayList;
import java.util.List;

// Recursive
// Time Complexity: O(n), n is the node number in the tree
// Space Complexity: O(h), h is the height of the tree
public class Solution1 {

    public List<Integer> preorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        preorderTraversal(root, res);
        return res;
    }

    private void preorderTraversal(TreeNode node, List<Integer> list){
        if(node != null){
            list.add(node.val);
            preorderTraversal(node.left, list);
            preorderTraversal(node.right, list);
        }
    }
}