二叉树的前序遍历

package leetcode.wyj;

import leetcode.model.TreeNode;

import java.util.ArrayList;
import java.util.List;

/**
 * 给你二叉树的根节点 root ，返回它节点值的 前序 遍历。
 * <p>
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode() {}
 * TreeNode(int val) { this.val = val; }
 * TreeNode(int val, TreeNode left, TreeNode right) {
 * this.val = val;
 * this.left = left;
 * this.right = right;
 * }
 * }
 */
public class LeetCode144 {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        pre(result, root);
        return result;
    }

    public void pre(List<Integer> result, TreeNode node) {
        if (node == null) {
            return;
        }
        result.add(node.val);
        if (node.left != null) {
            pre(result, node.left);
        }
        if (node.right != null) {
            pre(result, node.right);
        }
    }
}