BinaryTreePreorderTraversal.java

package gitLeetCode;

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

/**
 * 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].
 *
 * @author chenshuna
 *
 *         Note: Recursive solution
 */

public class BinaryTreePreorderTraversal {
    /**
     * Recursive solution
     */
    public static List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root == null) {
            return res;
        }
        preorderTraversalTree(root, res);
        return res;
    }

    public static void preorderTraversalTree(TreeNode root, List<Integer> res) {
        if (root != null) {
            res.add(root.val);
            preorderTraversalTree(root.left, res);
            preorderTraversalTree(root.right, res);
        } else return;
    }

    /**
     * iterative solution
     * Use stack
     */
    public static List<Integer> preorderTraversalIterative(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root == null) {
            return res;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            res.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        return res;
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        TreeNode res = new TreeNode(0);
        res.left = new TreeNode(2);
        res.right = new TreeNode(1);
        res.left.left = new TreeNode(4);
        res.left.right = new TreeNode(5);
        res.left.left.left = new TreeNode(9);

        System.out.print(preorderTraversal(res));
        System.out.print(preorderTraversalIterative(res));
    }

}