二叉树的前序遍历
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
输出: [1,2,3]
来源:力扣(LeetCode)
链接:https://leetcode-cn.com/problems/binary-tree-preorder-traversal
递归实现:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { public List<Integer> preorderTraversal(TreeNode root) { List<Integer> list = new ArrayList<>(); if(root ==null) return list; help(root,list); return list; } public List<Integer> help(TreeNode node ,List<Integer> list){ if(node==null )return list; list.add(node.val); help(node.left,list); help(node.right,list); return list; } }
迭代实现:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { public List<Integer> preorderTraversal(TreeNode root) { List<Integer> list = new ArrayList<Integer>(); Stack<TreeNode> stack = new Stack<>(); if(root == null) return list; stack.push(root); while(!stack.isEmpty()){ root = stack.pop(); list.add(root.val); if(root.right != null) stack.push(root.right); if(root.left != null) stack.push(root.left); } return list; } }
但是相对与迭代实现,递归实现相对简单而且效率较高。