144. Binary Tree Preorder Traversal

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LeetCode

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Problem

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

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

Code

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root == null) return res;

        helper(res, root);
        return res;
    }

    private void helper(List<Integer> res, TreeNode root){
        if(root == null) return;
        res.add(root.val);
        helper(res, root.left);
        helper(res, root.right);
    }
}

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root == null) return res;

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            if(cur.right != null) stack.push(cur.right);
            if(cur.left != null) stack.push(cur.left);
            res.add(cur.val);
        }

        return res;
    }
}