144.Binary Tree Preorder Traversal

Given a binary tree, return thepreordertraversal of its nodes' values.

For example:
Given binary tree[1,null,2,3],

   1
    \
     2
    /
   3

return[1,2,3].

Note:Recursive solution is trivial, could you do it iteratively?

Solution)

Iterative method

/**
 * 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<>();
        if (root != null)
            stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode node = stack.pop();
            list.add(node.val);
            if (node.right != null)
                stack.push(node.right);
            if (node.left != null)
                stack.push(node.left);
        }
        return list;
    }
}

Recursive method

class Solution {
    public void preorderTraversalHelper(TreeNode root, List<Integer> list) {
        if (root == null) return;
        list.add(root.val);
        preorderTraversalHelper(root.left, list);
        preorderTraversalHelper(root.right, list);
    }
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        preorderTraversalHelper(root, list);
        return list;
    }
}