144. 前序遍歷 Leetcode Java

題目：

給定一個二叉樹，返回它的 前序 遍歷。

 示例:

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

輸出: [1,2,3]
進階: 遞歸算法很簡單，你可以通過迭代算法完成嗎？

遞歸法：

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        helper(root, res);
        return res;
    }

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

迭代法：

利用棧

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {


        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        if(root==null) return res;
        stack.push(root);
        //先進後出
        while (!stack.isEmpty()) {
            TreeNode curr =  stack.pop();
            res.add(curr.val);
            if(curr.right!=null){
                stack.push(curr.right);
            }
            if(curr.left!=null){
                stack.push(curr.left);
            }

        }
        return res;
    }

}