二叉树的前序遍历，两种方法（递归 / 迭代）

leetcode 144. 二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历

从根节点开始先走左子树 再走右子树

示例:

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

输出: [1,2,3]

//递归法
/**
 * 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<>();
        if(root == null) {
            return list;
        }
        TreeNode cur = root;
        list.add(cur.val);
        List<Integer> left = preorderTraversal(cur.left);
        list.addAll(left);
        List<Integer> right = preorderTraversal(cur.right);
        list.addAll(right);
        return list;
    }
}

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