[leetCode]144. 二叉树的前序遍历

csdn:https://blog.csdn.net/renweiyi1487/article/details/109308919

题目

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

给定一个二叉树，返回它的 前序 遍历。

 示例:

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

递归

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

    private void traverse(TreeNode cur, List<Integer> ans) {
        if (cur == null) {
            return;
        }
        ans.add(cur.val);
        traverse(cur.left, ans);
        traverse(cur.right, ans);
    }
}

迭代

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        if (root == null)
            return ans;
        Deque<TreeNode> stack = new LinkedList<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node != null) {
                if (node.right != null)
                    stack.push(node.right); // 右
                if (node.left != null) 
                    stack.push(node.left);  // 左
                stack.push(node);
                stack.push(null); //  中，要处理的节点进行标记
            } else {
                node = stack.pop();
                ans.add(node.val);
            }
        }
        return ans;
    }
}