Leetcode 94. 二叉树的中序遍历

题目链接

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

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

采用栈来遍历，一直遍历到最左节点，然后弹出该节点，把该节点的右子节点加入栈中。

代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if (root == null) { return list; }
        LinkedList<TreeNode> stack = new LinkedList<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
            while (cur != null) {
                stack.push(cur);
                cur = cur.left;
            }
            cur = stack.pop();
            list.add(cur.val);
            cur = cur.right;
        }
        return list;
    }
}