题意：
给定二叉树，返回前序遍历的值
分析：
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { List<Integer> list = new ArrayList<>(); public List<Integer> preorderTraversal(TreeNode root) { helper(root); return list; } private void helper(TreeNode root){ if(root == null) return; list.add(root.val); helper(root.left); helper(root.right); } } 迭代深搜： /** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public List<Integer> preorderTraversal(TreeNode root) { Stack<TreeNode> stack = new Stack<>(); List<Integer> list = new ArrayList<>(); if(root!=null){ stack.push(root); while(!stack.isEmpty()){ TreeNode node = stack.pop(); list.add(node.val); if(node.right!=null){ stack.push(node.right); } if(node.left!=null){ stack.push(node.left); } } } return list; } }