[144. Binary Tree Preorder Traversal] #leetcode

144_0822.java

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode curNode = root;
        while ( !stack.isEmpty() || curNode != null) {
            if (curNode != null) {
                result.add(curNode.val);
                stack.push(curNode);
                curNode = curNode.left;
            } else {
                curNode = stack.pop().right;
            } 
        }
        
        return result;
    }
}