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LeetCode 144. 二叉树的前序遍历

年迈的代码机器 2022-04-04 阅读 74

给你二叉树的根节点 root ,返回它节点值的 前序 遍历。

示例 1:


输入:root = [1,null,2,3]
输出:[1,2,3]
示例 2:

输入:root = []
输出:[]
示例 3:

输入:root = [1]
输出:[1]
示例 4:


输入:root = [1,2]
输出:[1,2]
示例 5:


输入:root = [1,null,2]
输出:[1,2]
 

提示:

树中节点数目在范围 [0, 100] 内
-100 <= Node.val <= 100

题解:

/**
 * 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) {
        List<Integer> list = new ArrayList<>();
        if (root == null) {
            return list;
        }
        list.add(root.val);
        List<Integer> listLeft = preorderTraversal(root.left);
        list.addAll(listLeft);
        List<Integer> listRight = preorderTraversal(root.right);
        list.addAll(listRight);
        return list;
    }
}
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