【二叉树】Leetcode 94. 二叉树的中序遍历【简单】

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。

示例 1：

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输入：root = [1,null,2,3]
输出：[1,3,2]

解题思路

中序遍历是一种二叉树遍历方式，按照“左根右”的顺序遍历二叉树节点。

1、递归地遍历左子树。
2、访问当前节点。
3、递归地遍历右子树。

对应先序遍历 根左右
对应后序遍历 左右根

先、中、后序遍历其实指的是遍历根节点的先后顺序

Java实现中序遍历

public class InorderTraversal {static class TreeNode {int val;TreeNode left;TreeNode right;TreeNode(int val) {this.val = val;}}public List<Integer> inorderTraversal(TreeNode root) {List<Integer> result = new ArrayList<>();inorderTraversalHelper(root, result);return result;}private void inorderTraversalHelper(TreeNode node, List<Integer> result) {if (node == null) {return;}inorderTraversalHelper(node.left, result);result.add(node.val);inorderTraversalHelper(node.right, result);}public static void main(String[] args) {// 示例用法TreeNode root = new TreeNode(1);root.left = new TreeNode(4);root.right = new TreeNode(2);root.right.left = new TreeNode(3);InorderTraversal solution = new InorderTraversal();List<Integer> result = solution.inorderTraversal(root);System.out.println(result);  // 输出：[4, 1, 3, 2]}
}

Java实现先序遍历

/*** 先序遍历* 根->左->右*/
public class PreorderTraversal {static class TreeNode {int val;TreeNode left;TreeNode right;TreeNode(int val) {this.val = val;}}public List<Integer> preorderTraversal(TreeNode root) {List<Integer> result = new ArrayList<>();preorderTraversalHelper(root, result);return result;}private void preorderTraversalHelper(TreeNode node, List<Integer> result) {if (node == null) {return;}result.add(node.val);preorderTraversalHelper(node.left,result);preorderTraversalHelper(node.right,result);}public static void main(String[] args) {// 示例用法TreeNode root = new TreeNode(1);root.left = new TreeNode(4);root.left.left = new TreeNode(5);root.left.left.right = new TreeNode(8);root.left.right = new TreeNode(6);root.right = new TreeNode(2);root.right.left = new TreeNode(3);PreorderTraversal solution = new PreorderTraversal();List<Integer> result = solution.preorderTraversal(root);System.out.println(result);  // 输出：[1, 3, 2]}
}

Java实现后序遍历

/*** 后序遍历* 左->右->根*/
public class PostorderTraversal {static class TreeNode {int val;TreeNode left;TreeNode right;TreeNode(int val) {this.val = val;}}public List<Integer> postorderTraversal(TreeNode root) {List<Integer> result = new ArrayList<>();postorderTraversalHelper(root, result);return result;}private void postorderTraversalHelper(TreeNode node, List<Integer> result) {if (node == null) {return;}postorderTraversalHelper(node.left, result);postorderTraversalHelper(node.right, result);result.add(node.val);}public static void main(String[] args) {// 示例用法TreeNode root = new TreeNode(1);root.left = new TreeNode(4);root.right = new TreeNode(2);root.right.left = new TreeNode(3);PostorderTraversal solution = new PostorderTraversal();List<Integer> result = solution.postorderTraversal(root);System.out.println(result);  // 输出：[1, 3, 2]}
}