https://oj.leetcode.com/problems/binary-tree-preorder-traversal/

Binary Tree Preorder Traversal.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        if (root != null){
            ret.add(root.val);
            if (root.left != null) ret.addAll(preorderTraversal(root.left));
            if (root.right != null) ret.addAll(preorderTraversal(root.right));
        }
        return ret;
    }
}