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

public class Solution {

    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public List<Integer> preorderTraversal(TreeNode root) {

        List<
 
Integer> res = new LinkedList<>();
        if(root == null)
            return res;

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            res.add(cur.val);

            if(cur.
 
right != null)
                stack.push(cur.right);
            if(cur.left != null)
                stack.push(cur.left);
        }

        return res;
    }
    }

方法二（採用遞迴方式）：

public class Solution {

    // Definition for a binary tree node.
    public class TreeNode {
        int val;
        TreeNode left;
 

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

    public List<Integer> preorderTraversal(TreeNode root) {

        List<Integer> res = new LinkedList<>();
        if(root == null)
            return null;

        System.out.println(root.val);
        res.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);
        return res;
    }