✨ new post: leetcode p94

Author: jooncco

_posts/2022-09-08-leetcode-94.md

@@ -0,0 +1,111 @@
+---
+title: "[Leetcode] 94. Binary Tree Inorder Traversal explained"
+excerpt: "Leetcode daily challenge 2022 september 8th solution"
+header:
+  overlay_image: /public/images/problem-solving-common-header.png
+tags:
+  - Tree
+  - Inorder traversal
+last_modified_at: 2022-09-08T11:07:08+09:00
+---
+
+<a href="https://leetcode.com/">
+    <img src="/public/images/leetcode-logo.jpeg"/>
+</a>
+
+## Problem
+
+<a href="https://leetcode.com/problems/binary-tree-inorder-traversal/">
+    <img src="/public/images/leetcode-94.png"/>
+</a>
+
+<br/>
+
+## Key Idea
+
+Recursive approach is trivial.  
+We can implement inorder traversal using `Stack`.
+
+```python
+curNode <- root
+while true:
+    if curNode is null:
+        if stack is empty:
+            break;
+        tmpNode <- stack.pop();
+        add tmpNode.val to result
+        curNode <- tmpNode.right;
+    else:
+        if curNode is not null:
+            push curNode into stack
+        else:
+            curNode <- curNode.left;
+```
+
+- Time: $$O(n)$$
+- Space: $$O(n)$$
+
+<br/>
+
+## Implementation
+
+<img src="/public/images/leetcode-94-result.png"/>
+
+### Recursive approach
+
+```java
+/**
+ * author: jooncco
+ * written: 2022. 9. 8. Tue. 11:34:14 [UTC+9]
+ **/
+
+class Solution {
+    public List<Integer> inorderTraversal(TreeNode root) {
+        List<Integer> result = new LinkedList<>();
+        inorder(result, root);
+        return result;
+    }
+
+    private void inorder(List<Integer> arr, TreeNode node) {
+        if (node == null) return;
+
+        inorder(arr, node.left);
+        arr.add(node.val);
+        inorder(arr, node.right);
+    }
+}
+```
+
+### Iterative approach
+
+```java
+/**
+ * author: jooncco
+ * written: 2022. 9. 8. Tue. 13:36:14 [UTC+9]
+ **/
+
+class Solution {
+    public List<Integer> inorderTraversal(TreeNode root) {
+        Stack<TreeNode> stack = new Stack<>();
+        List<Integer> result= new LinkedList<>();
+
+        TreeNode curNode = root;
+        while (true) {
+            if (curNode == null) {
+                if (stack.isEmpty()) break;
+                result.add(stack.peek().val);
+                curNode= stack.pop().right;
+            } else {
+                if (curNode.left != null) {
+                    stack.push(curNode);
+                    curNode= curNode.left;
+                } else {
+                    result.add(curNode.val);
+                    curNode= curNode.right;
+                }
+            }
+        }
+        return result;
+    }
+}
+```