LeetCode-144 Binary Tree Preorder Traversal Solution (with Java)

1. Description:

2.Solutions:

/**
 * Created by sheepcore on 2019-05-09
 * 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<>();
        Stack<TreeNode> stack = new Stack<>();
        while(root != null || !stack.isEmpty()){
            while(root != null){
                list.add(root.val);
                stack.push(root);
                root = root.left;
            }
             if(!stack.isEmpty()) {
                root = stack.pop();
                root = root.right;
            } else
                return list;
        }
        return list;
    }
}