https://leetcode.com/problems/binary-tree-preorder-traversal/
题意
二叉树前序遍历，递归的比较简单，非递归实现需要用栈存节点
class Solution { List<Integer> results = new ArrayList<>(); Stack<TreeNode> stacks = new Stack<>(); public List<Integer> preorderTraversal(TreeNode root) { if (root == null) { return results; } //根节点入栈 stacks.push(root); while (!stacks.isEmpty()) { //出栈 TreeNode current = stacks.pop(); //加入结果集 results.add(current.val); //将右节点加入stack，之后遍历 if (current.right != null) { stacks.push(current.right); } //将左节点加入stack，在右节点之前遍历 if (current.left != null) { stacks.push(current.left); } } return results; } }