Leetcode(Java)-94. 二叉树的中序遍历

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？

思路1：递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> res =new ArrayList<>();
    public List<Integer> inorderTraversal(TreeNode root) {
        helper(root);
        return res;
    }
    private void helper(TreeNode root){
        if(root==null)
            return;
        
        helper(root.left);
        res.add(root.val);
        helper(root.right);
    }
}

思路2：用栈迭代

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