二叉树的前序遍历

二叉树的前序遍历

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

public class BinaryTreePreorderTraversal {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        method(root, list);
        return list;
    }

    /**
     * 递归实现
     *
     * @param root
     * @param list
     */
    private void method(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }

        method(root.left, list);

        method(root.right, list);
    }

    /**
     * 迭代实现
     * @param root
     * @param list
     */
    private void method1(TreeNode root, List<Integer> list) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.add(root);
        while (!stack.isEmpty()) {
            TreeNode head = stack.pop();
            list.add(head.val);
            if (head.right != null) {
                stack.add(head.right);
            }
            if (head.left != null) {
                stack.add(head.left);
            }

        }

    }
}