144. Binary Tree Preorder Traversal

Solution 1://非递归

public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> result = new ArrayList<>();
    if (root == null) {
        return result;
    }
    Stack<TreeNode> stack = new Stack<>();
    stack.push(root);
    while (!stack.empty()) {
        TreeNode node = stack.pop();
        result.add(node.val);
        if (node.right != null) {
            stack.push(node.right);
        }
        if (node.left != null) {
            stack.push(node.left);
        }
    }
    return result;
}

Solution 2：//递归

public class Solution {
    List<Integer> result = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        if (root != null)
            helper(root);
        return result;
    }
    private void helper(TreeNode root) {
        result.add(root.val);
        if (root.left != null)
            helper(root.left);
        if (root.right != null)
            helper(root.right);
    }
}