131、二叉树的前序遍历

题目描述：

使用非递归的算法实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
      
		List<Integer> list = new ArrayList<>();
		if(root != null){
			Stack<TreeNode> queue = new Stack();
			queue.push(root);
			while (!queue.isEmpty()) {
				TreeNode node = queue.pop();
				list.add(node.val);
				if(node.right != null){
					queue.push(node.right);
				}
				if(node.left != null){
					queue.push(node.left);
				}
			}
		}
		return list;  
    }
}