题目地址

https://leetcode-cn.com/problems/binary-tree-inorder-traversal/

题目描述

给定一个二叉树，返回它的中序 遍历。

示例:

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,3,2]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

题解

递归解法

/**
 * 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> results = new ArrayList<>();
        traverse(root, results);
        return results;
    }

    public void traverse(TreeNode root, List<Integer> results) {
        if (root == null) return;

        traverse(root.left, results);
    
        // 中序遍历
        results.add(root.val);
    
        traverse(root.right, results);
    }
}

迭代解法

 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<> ();
        Stack<TreeNode> stack = new Stack<> ();

        while (root != null || !stack.isEmpty()) {
            // 左子树入栈
            while (root != null) {
                stack.push(root);
                root = root.left;
            }

            // 中序遍历
            root = stack.pop();
            res.add(root.val);

            // 遍历右子树
            root = root.right;
        }

        return res;
    }
}

参考

• https://leetcode-solution-leetcode-pp.gitbook.io/leetcode-solution/medium/94.binary-tree-inorder-traversal