LeetCode 二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历。
示例：

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [1,2,3]

非递归（思路最清晰）：

/**
 * 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<>();
        TreeNode cur = root;
        if(cur == null) return list;
        while(cur != null){
            list.add(cur.val);
            stack.push(cur);
            cur = cur.left;
        }
        while(!stack.isEmpty()){
            cur = stack.pop();
            cur = cur.right;
            while(cur != null){
                list.add(cur.val);
                stack.push(cur);
                cur = cur.left;
            }
        }
        return list;
    }
    
}

非递归（上一版本的改进，精简了代码）：

/**
 * 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<>();
        TreeNode cur = root;
        while(cur != null || !stack.isEmpty()){
            if(cur != null){
                stack.push(cur);
                list.add(cur.val);
                cur=cur.left;
            }else{
                cur = stack.pop();
                cur = cur.right;
            }
        }
        return list;
    }
    
}

递归：

/**
 * 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<>();
        return test(root,list);
        
    }
    List<Integer> test(TreeNode root, List<Integer> list){
        if(root != null){
            list.add(root.val);
            test(root.left,list);
            test(root.right,list);
        }
        return list;
    }
}