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# 94 Binary Tree Inorder Traversal

## 94. [Binary Tree Inorder Traversal](https://leetcode.com/problems/binary-tree-inorder-traversal/description/)

## 1. Question

Given a binary tree, return the inorder traversal of its nodes' values.

For example:\
Given binary tree`[1,null,2,3]`,

```
   1
    \
     2
    /
   3
```

return`[1,3,2]`.

**Note:** Recursive solution is trivial, could you do it iteratively?

## 2. Implementation

```java
public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
         List<Integer> res = new ArrayList<>();

        // Method 1: Iterative Stack
        TreeNode curNode = root;
        Stack<TreeNode> stack = new Stack<>();

        while (curNode != null || !stack.isEmpty()) {
            if (curNode != null) {
                stack.push(curNode);
                curNode = curNode.left;
            }
            else {
                curNode = stack.pop();
                res.add(curNode.val);
                curNode = curNode.right;
            }
        }
        return res;
    }
}
```

## 3. Time & Space Complexity

时间空间都是O(n)