【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：

在这里插入图片描述
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]
    }
}