144. Binary Tree Preorder Traversal

题目：

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

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,2,3].

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

链接：https://leetcode.com/problems/binary-tree-preorder-traversal/#/description

5/10/2017

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

        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode current;
        stack.push(root);
        while (!stack.empty()) {
            current = stack.pop();
            if (current.right != null) {
                stack.push(current.right);
            }
            if (current.left != null) {
                stack.push(current.left);
            }
            ret.add(current.val);
        }
        return current;
    }
}

更多讨论

https://discuss.leetcode.com/category/152/binary-tree-preorder-traversal

需要再看morris traversal