LeetCode--Binary Tree Preorder Traversal

最新推荐文章于 2025-08-11 00:49:14 发布

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最新推荐文章于 2025-08-11 00:49:14 发布

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分类专栏： LeetCode 文章标签： Interview leetcode Tree

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<span style="font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif; line-height: 1.1; background-color: rgb(255, 255, 255);">Binary Tree Preorder Traversal</span>

与其他树的遍历思路相同，递归或者栈，时间空间复杂度也没有不同。不多说，代码如下：

recursion:

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        preorderTraversalHelper(root, list);
        return list;
    }
    
    public void preorderTraversalHelper(TreeNode root, List<Integer> list) {
        if(root == null) {
            return;
        }
        
        list.add(root.val);
        preorderTraversalHelper(root.left, list);
        preorderTraversalHelper(root.right, list);
    }
}

Stack：

</pre><pre name="code" class="java">public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        if(root == null) {
            return list;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while(root != null || !stack.isEmpty()) {
            if(root != null) {
                stack.push(root);
                list.add(root.val);
                root = root.left;
            } else {
                TreeNode node = stack.pop();
                root = node.right;
            }
        }
        return list;
    }
}