bst preorder.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) {
        List<Integer> res = new ArrayList<Integer>();
        if (root == null)
            return res;
        TreeNode curr = root;
        Stack<TreeNode> path = new Stack<TreeNode>();
        while(!path.isEmpty() || curr != null) {
            while (curr != null) {
                path.push(curr);
                res.add(curr.val);
                curr = curr.left;
            }
            curr = path.pop();
            curr = curr.right;
        }
        return res;
    }
}