题解 | #二叉树的前序遍历#

import java.util.*;

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) {
        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.add(root);
        while(!stack.isEmpty()){
            TreeNode pop = stack.pop();
            if(pop!=null){
                result.add(pop.val);
                stack.add(pop.right);
                stack.add(pop.left);
            }
        }
        int[] r = new int[result.size()];
        for (int i = result.size() - 1; i >= 0; i--) {
            r[i] = result.get(i);
        }
        return r;
    }
}