二叉树的前序遍历
https://leetcode.cn/problems/binary-tree-preorder-traversal/
给你二叉树的根节点 root ，返回它节点值的 前序 遍历。
示例 1：
输入：root = [1,null,2,3]
输出：[1,2,3]
示例 2：
输入：root = []
输出：[]
示例 3：
输入：root = [1]
输出：[1]
示例 4：
输入：root = [1,2]
输出：[1,2]
示例 5：
输入：root = [1,null,2]
输出：[1,2]
提示：
树中节点数目在范围 [0, 100] 内
-100 <= Node.val <= 100
递归
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- 自己调用自己
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> list=new ArrayList<>();
if(root!=null){
fun(root,list);
}
return list;
}
public static void fun(TreeNode root,List<Integer> list){
if(root!=null){
list.add(root.val);
if(root.left!=null)
fun(root.left,list);
if(root.right!=null)
fun(root.right,list);
}
}
}
二叉树的中序遍历
给定一个二叉树的根节点 root ，返回 它的 中序 遍历 。
递归
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> list=new ArrayList<>();
fun(root,list);
return list;
}
public static void fun(TreeNode root,List<Integer> list) {
if (root != null) {
if (root.left != null)
fun(root.left, list);
list.add(root.val);
if (root.right != null)
fun(root.right, list);
}
}
}
二叉树的后序遍历
递归
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> list=new ArrayList<>();
fun(root,list);
return list;
}
public static void fun(TreeNode root,List<Integer> list) {
if (root != null) {
if (root.left != null)
fun(root.left, list);
if (root.right != null)
fun(root.right, list);
list.add(root.val);
}
}
}