Aug 31, 2025 · Aug 24, 2025 · Aug 28, 2025 · Aug 31, 2025
diff --git a/articles/construct-binary-tree-from-inorder-and-postorder-traversal.md b/articles/construct-binary-tree-from-inorder-and-postorder-traversal.md
 }
 ```

 ```csharp
 /**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
 public class Solution {
    public TreeNode BuildTree(int[] inorder, int[] postorder) {
        if (postorder.Length == 0 || inorder.Length == 0) {
            return null;
        }

        int rootVal = postorder[postorder.Length - 1];
        TreeNode root = new TreeNode(rootVal);

        int mid = Array.IndexOf(inorder, rootVal);

        int[] leftInorder = inorder[..mid];
        int[] rightInorder = inorder[(mid + 1)..];

        int[] leftPostorder = postorder[..mid];
        int[] rightPostorder = postorder[mid..^1];

        root.left = BuildTree(leftInorder, leftPostorder);
        root.right = BuildTree(rightInorder, rightPostorder);

        return root;
    }
 }
 ```

 ::tabs-end

 ### Time & Space Complexity
 }
 ```

 ```csharp
 /**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
 public class Solution {
    int[] postorder;
    Dictionary<int, int> inorderIdx;
    int idx;

    public TreeNode BuildTree(int[] inorder, int[] postorder) {
        this.postorder = postorder;
        inorderIdx = new Dictionary<int, int>();
        for (int i = 0; i < inorder.Length; i++) {
            inorderIdx[inorder[i]] = i;
        }
        idx = postorder.Length - 1;
        return Dfs(0, inorder.Length - 1);
    }

    private TreeNode Dfs(int l, int r) {
        if (l > r) return null;

        TreeNode root = new TreeNode(postorder[idx--]);
        int i = inorderIdx[root.val];
        root.right = Dfs(i + 1, r);
        root.left = Dfs(l, i - 1);
        return root;
    }
 }
 ```

 ::tabs-end

 ### Time & Space Complexity
 }
 ```

 ```csharp
 /**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
 public class Solution {
    int postIdx, inIdx;
    int[] inorder, postorder;

    public TreeNode BuildTree(int[] inorder, int[] postorder) {
        this.inorder = inorder;
        this.postorder = postorder;
        postIdx = inIdx = postorder.Length - 1;
        return Dfs(int.MaxValue);
    }

    private TreeNode Dfs(int limit) {
        if (postIdx < 0) return null;
        if (inorder[inIdx] == limit) {
            inIdx--;
            return null;
        }

        TreeNode root = new TreeNode(postorder[postIdx--]);
        root.right = Dfs(root.val);
        root.left = Dfs(limit);
        return root;
    }
 }
 ```

 ::tabs-end

 ### Time & Space Complexity

 - Time complexity: $O(n)$
 - Space complexity: $O(n)$ for recursion stack.
 - Space complexity: $O(n)$ for recursion stack.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -142,6 +142,45 @@ class Solution {
		}
		```

		```csharp
		/**
		* Definition for a binary tree node.
		* public class TreeNode {
		* public int val;
		* public TreeNode left;
		* public TreeNode right;
		* public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
		* this.val = val;
		* this.left = left;
		* this.right = right;
		* }
		* }
		*/
		public class Solution {
		public TreeNode BuildTree(int[] inorder, int[] postorder) {
		if (postorder.Length == 0 \|\| inorder.Length == 0) {
		return null;
		}

		int rootVal = postorder[postorder.Length - 1];
		TreeNode root = new TreeNode(rootVal);

		int mid = Array.IndexOf(inorder, rootVal);

		int[] leftInorder = inorder[..mid];
		int[] rightInorder = inorder[(mid + 1)..];

		int[] leftPostorder = postorder[..mid];
		int[] rightPostorder = postorder[mid..^1];

		root.left = BuildTree(leftInorder, leftPostorder);
		root.right = BuildTree(rightInorder, rightPostorder);

		return root;
		}
		}
		```

		::tabs-end

		### Time & Space Complexity
Expand DownExpand Up		@@ -301,6 +340,47 @@ class Solution {
		}
		```

		```csharp
		/**
		* Definition for a binary tree node.
		* public class TreeNode {
		* public int val;
		* public TreeNode left;
		* public TreeNode right;
		* public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
		* this.val = val;
		* this.left = left;
		* this.right = right;
		* }
		* }
		*/
		public class Solution {
		int[] postorder;
		Dictionary<int, int> inorderIdx;
		int idx;

		public TreeNode BuildTree(int[] inorder, int[] postorder) {
		this.postorder = postorder;
		inorderIdx = new Dictionary<int, int>();
		for (int i = 0; i < inorder.Length; i++) {
		inorderIdx[inorder[i]] = i;
		}
		idx = postorder.Length - 1;
		return Dfs(0, inorder.Length - 1);
		}

		private TreeNode Dfs(int l, int r) {
		if (l > r) return null;

		TreeNode root = new TreeNode(postorder[idx--]);
		int i = inorderIdx[root.val];
		root.right = Dfs(i + 1, r);
		root.left = Dfs(l, i - 1);
		return root;
		}
		}
		```

		::tabs-end

		### Time & Space Complexity
Expand DownExpand Up		@@ -468,9 +548,49 @@ class Solution {
		}
		```

		```csharp
		/**
		* Definition for a binary tree node.
		* public class TreeNode {
		* public int val;
		* public TreeNode left;
		* public TreeNode right;
		* public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
		* this.val = val;
		* this.left = left;
		* this.right = right;
		* }
		* }
		*/
		public class Solution {
		int postIdx, inIdx;
		int[] inorder, postorder;

		public TreeNode BuildTree(int[] inorder, int[] postorder) {
		this.inorder = inorder;
		this.postorder = postorder;
		postIdx = inIdx = postorder.Length - 1;
		return Dfs(int.MaxValue);
		}

		private TreeNode Dfs(int limit) {
		if (postIdx < 0) return null;
		if (inorder[inIdx] == limit) {
		inIdx--;
		return null;
		}

		TreeNode root = new TreeNode(postorder[postIdx--]);
		root.right = Dfs(root.val);
		root.left = Dfs(limit);
		return root;
		}
		}
		```

		::tabs-end

		### Time & Space Complexity

		- Time complexity: $O(n)$
		- Space complexity: $O(n)$ for recursion stack.
		- Space complexity: $O(n)$ for recursion stack.