题解 | #二叉树的前序遍历#

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 int[] preorderTraversal (TreeNode root) {
        if(root == null){
            return new int[]{};
        }
        // write code here
        int[] left = preorderTraversal(root.left);
        int[] right = preorderTraversal(root.right);
        int [] result = new int[left.length + right.length + 1];
        result[0] = root.val;
        System.arraycopy(left, 0, result, 1, left.length);
        System.arraycopy(right, 0, result, left.length + 1, right.length);
        return result;
    }
}

二叉树前序遍历，分成左右两个分支然后递归。