【LeetCode】144. 二叉树的前序遍历

题目

https://leetcode.cn/problems/binary-tree-preorder-traversal/description/

思路

• 根放入
• 左都放入
• 右都放入

code

class Solution {
    List<Integer>list = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list= new ArrayList<>();
        if(root == null) return list;
        //根放入
        list.add(root.val);

        List<Integer> leftTree =preorderTraversal(root.left);
        list.addAll(leftTree);//左都放入
        List<Integer> rightTree = preorderTraversal(root.right);
        list.addAll(rightTree);//右都放入
        return list;
                
    }
}