leetcode-144. 二叉树的前序遍历

/**
 * 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;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        // 定义结果集
        List<Integer> res = new ArrayList<Integer>();
        // 从根节点开始递归
        preorder(root, res);
        // 返回结果集
        return res;
    }
    // 前序遍历
    public void preorder(TreeNode root, List<Integer> res) {
        // 判空
        if (root == null) {
            return;
        }
        // 将 root 节点的值加入结果集
        res.add(root.val);
        // 从左子树开始遍历
        preorder(root.left, res);
        // 右子树遍历
        preorder(root.right, res);
    }
}