給你二叉樹的根節點 root ，返回它節點值的前序遍歷。

來源：力扣（LeetCode）
連結：https://leetcode-cn.com/problems/binary-tree-preorder-traversal
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import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        if (root == null) {
            return Collections.emptyList();
        }

        List<Integer> ret = new ArrayList<>();

        TreeNode cur = root;
        while (cur != null) {
            TreeNode mostRight = cur.left;
            if (mostRight != null) {
                while (mostRight.right != null && mostRight.right != cur) {
                    mostRight = mostRight.right;
                }
                if (mostRight.right == null) {
                    ret.add(cur.val);
                    mostRight.right = cur;
                    cur = cur.left;
                    continue;
                } else {
                    mostRight.right = null;
                }
            } else {
                ret.add(cur.val);
            }
            cur = cur.right;
        }


        return ret;
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}