Iterative Binary Tree Pre-order Traversal

IterativeBinaryTreePreOrder.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
      TreeNode node = root;
      Stack<TreeNode> stack = new Stack<TreeNode>();
      List<Integer> retList = new ArrayList<Integer>();
      while (true){
        if (node != null){
          retList.add(node.val);
          stack.push(node);
          node = node.left;
        }else{
          if (stack.empty()){
            return retList;
          }
          node = stack.pop();
          node = node.right;
        }
      }
    }
}