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;}
}

144. 二叉树的前序遍历 - 力扣（LeetCode）

/*** 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> res = new ArrayList<>();if (root == null) return res;traversal(res, root);return res;}private void traversal(List<Integer> res, TreeNode root) {if (root == null) return;res.add(root.val);traversal(res, root.left);traversal(res, root.right);}
}

145. 二叉树的后序遍历 - 力扣（LeetCode）

/*** 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> res = new ArrayList<>();if (root == null) return res;traversal(res, root);return res;}private void traversal(List<Integer> res, TreeNode root) {if (root == null) return;traversal(res, root.left);traversal(res, root.right);res.add(root.val);}
}

94. 二叉树的中序遍历 - 力扣（LeetCode）

/*** 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> res = new ArrayList<>();if (root == null) return res;traversal(res, root);return res;}private void traversal(List<Integer> res, TreeNode root) {if (root == null) return;traversal(res, root.left);res.add(root.val);traversal(res, root.right);}
}