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) {
        // write code here
        Stack<TreeNode> sta = new Stack<>();
        List<Integer> list = new ArrayList<>();
        if (root == null)
            return new int[0];
        sta.push(root);
        while (!sta.empty()) {
            TreeNode tmp = sta.pop();
            list.add(tmp.val);
            if (tmp.right != null)
                sta.push(tmp.right);
            if (tmp.left != null)
                sta.push(tmp.left);
        }
        int[] res = list.stream().mapToInt(Integer::intValue).toArray();
        return res;
    }
}