binary_tree_preorder_traversal_template.md

public class Solution {


    //模板
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        traversal(root, result);
        return result;
    }


    public void traversal(TreeNode root, ArrayList<Integer> result) {
        if (root == null) {
            return;
        }
        //do sth with the root(eg:print  visit  result.add(root.val);)  前序这一行
        traversal(root.left, result);
        //do sth with the root(eg:print  visit  result.add(root.val);) 中序在一样
        traversal(root.right, result);
        //do sth with the root(eg:print  visit  result.add(root.val);)  后序在者一样
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}