Method preorder

TreePreorder.java:2–8

(TreeNode root,List<Integer> list)

class Solution {
    public void preorder(TreeNode root,List<Integer> list)
    {
        if(root==null) return;
        list.add(root.val);
        preorder(root.left,list);
        preorder(root.right,list);
    }
    public List<Integer> preorderTraversal(TreeNode root) {
            List<Integer> list = new ArrayList<Integer>();
            preorder(root,list);