leetcode：144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},
return [1,2,3].

public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> valList = new ArrayList<>();
List<TreeNode> nodeList = new ArrayList<>();
while (root != null) {
valList.add(root.val);
if (root.right != null) {
nodeList.add(root.right);
}
if (root.left != null) {
root = root.left;
} else {
if (nodeList.size() > 0) {
root = nodeList.get(nodeList.size() - 1);
nodeList.remove(nodeList.size() - 1);
} else {
break;
}
}
}
return valList;
}
}