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) {
LinkedList<Integer> list = new LinkedList<>();
if (root == null){
return new int[0];
}
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while (!stack.isEmpty()){
//处理根节点
TreeNode node = stack.pop();
list.add(node.val);
//根左右,由于栈是先进后出,所以要让左节点后入栈,这样才能先弹出来
if (node.right!=null){
stack.push(node.right);
}
if (node.left != null){
stack.push(node.left);
}
}
return list.stream().mapToInt(Integer::intValue).toArray();
}
}