144. Binary Tree Preorder Traversal

题目地址：https://leetcode.com/problems/binary-tree-preorder-traversal/description/

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

Example:

Input: [1,null,2,3]
   1
    
     2
    /
   3

Output: [1,2,3]

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

java模版无法显示null，为了便于显示，改为c++模版

非递归(迭代版)：

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

递归版：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        preorder(list, root);
        return list;
    }
    public void preorder(List<Integer> list, TreeNode root) {        
        if (root == null) {
            return ;
        }
        list.add(root.val);
        preorder(list, root.left);
        preorder(list, root.right);
    }
}

Debug code in playground:

/* -----------------------------------
 *  WARNING:
 * -----------------------------------
 *  Your code may fail to compile
 *  because it contains public class
 *  declarations.
 *  To fix this, please remove the
 *  "public" keyword from your class
 *  declarations.
 */

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        preorder(list, root);
        return list;
    }
    public void preorder(List<Integer> list, TreeNode root) {        
        if (root == null) {
            return ;
        }
        list.add(root.val);
        preorder(list, root.left);
        preorder(list, root.right);
    }
}

public class MainClass {
    public static TreeNode stringToTreeNode(String input) {
        input = input.trim();
        input = input.substring(1, input.length() - 1);
        if (input.length() == 0) {
            return null;
        }
    
        String[] parts = input.split(",");
        String item = parts[0];
        TreeNode root = new TreeNode(Integer.parseInt(item));
        Queue<TreeNode> nodeQueue = new LinkedList<>();
        nodeQueue.add(root);
    
        int index = 1;
        while(!nodeQueue.isEmpty()) {
            TreeNode node = nodeQueue.remove();
    
            if (index == parts.length) {
                break;
            }
    
            item = parts[index++];
            item = item.trim();
            if (!item.equals("null")) {
                int leftNumber = Integer.parseInt(item);
                node.left = new TreeNode(leftNumber);
                nodeQueue.add(node.left);
            }
    
            if (index == parts.length) {
                break;
            }
    
            item = parts[index++];
            item = item.trim();
            if (!item.equals("null")) {
                int rightNumber = Integer.parseInt(item);
                node.right = new TreeNode(rightNumber);
                nodeQueue.add(node.right);
            }
        }
        return root;
    }
    
    public static String integerArrayListToString(List<Integer> nums, int length) {
        if (length == 0) {
            return "[]";
        }
    
        String result = "";
        for(int index = 0; index < length; index++) {
            Integer number = nums.get(index);
            result += Integer.toString(number) + ", ";
        }
        return "[" + result.substring(0, result.length() - 2) + "]";
    }
    
    public static String integerArrayListToString(List<Integer> nums) {
        return integerArrayListToString(nums, nums.size());
    }
    
    public static void main(String[] args) throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String line;
        while ((line = in.readLine()) != null) {
            TreeNode root = stringToTreeNode(line);
            
            List<Integer> ret = new Solution().preorderTraversal(root);
            
            String out = integerArrayListToString(ret);
            
            System.out.print(out);
        }
    }
}

========================================Talk is cheap, show me the code=======================================