LeetCode 144. 二叉树的前序遍历(Binary Tree Preorder Traversal)

144. 二叉树的前序遍历
144. Binary Tree Preorder Traversal

题目描述
给定一个二叉树，返回它的 前序 遍历。

LeetCode144. Binary Tree Preorder Traversal

示例:
输入: [1,null,2,3]

   1
    
     2
    /
   3 

输出: [1,2,3]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

Java 实现
Iterative Solution

import java.util.LinkedList;
import java.util.List;
import java.util.Stack;

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

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

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new LinkedList<>();
        if (root == null) {
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
            result.add(node.val);
        }
        return result;
    }
}

Recursive Solution

import java.util.LinkedList;
import java.util.List;

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

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

class Solution {
    List<Integer> result = new LinkedList<>();

    public List<Integer> preorderTraversal(TreeNode root) {
        if (root == null) {
            return result;
        }
        result.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);
        return result;
    }
}

相似题目

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• 589. N叉树的前序遍历

参考资料

• https://leetcode-cn.com/problems/binary-tree-preorder-traversal/
• https://leetcode.com/problems/binary-tree-preorder-traversal/