题解 | 二叉树的前序遍历

递归

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public static void preorder(List<Integer> list,TreeNode root){
        if(root==null){
            return ;
        }
        list.add(root.val);
        preorder(list,root.left);
        preorder(list,root.right);

    }
    public int[] preorderTraversal (TreeNode root) {
        // write code here
        List<Integer> list=new ArrayList<>();
        preorder(list,root);
        int n=list.size();
        int[] result=new int[n];
        for(int i=0;i<n;i++){
            result[i]=list.get(i);


        }
        return result;
    }
}