@@ -142,6 +142,45 @@ class Solution {
142142}
143143```
144144
145+ ``` csharp
146+ /**
147+ * Definition for a binary tree node.
148+ * public class TreeNode {
149+ * public int val;
150+ * public TreeNode left;
151+ * public TreeNode right;
152+ * public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
153+ * this.val = val;
154+ * this.left = left;
155+ * this.right = right;
156+ * }
157+ * }
158+ */
159+ public class Solution {
160+ public TreeNode BuildTree (int []inorder ,int []postorder ) {
161+ if (postorder .Length == 0 || inorder .Length == 0 ) {
162+ return null ;
163+ }
164+
165+ int rootVal = postorder [postorder .Length - 1 ];
166+ TreeNode root = new TreeNode (rootVal );
167+
168+ int mid = Array .IndexOf (inorder ,rootVal );
169+
170+ int []leftInorder = inorder [.. mid ];
171+ int []rightInorder = inorder [(mid + 1 ).. ];
172+
173+ int []leftPostorder = postorder [.. mid ];
174+ int []rightPostorder = postorder [mid .. ^ 1 ];
175+
176+ root .left = BuildTree (leftInorder ,leftPostorder );
177+ root .right = BuildTree (rightInorder ,rightPostorder );
178+
179+ return root ;
180+ }
181+ }
182+ ```
183+
145184:: tabs-end
146185
147186###Time & Space Complexity
@@ -301,6 +340,47 @@ class Solution {
301340}
302341```
303342
343+ ``` csharp
344+ /**
345+ * Definition for a binary tree node.
346+ * public class TreeNode {
347+ * public int val;
348+ * public TreeNode left;
349+ * public TreeNode right;
350+ * public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
351+ * this.val = val;
352+ * this.left = left;
353+ * this.right = right;
354+ * }
355+ * }
356+ */
357+ public class Solution {
358+ int []postorder ;
359+ Dictionary <int ,int >inorderIdx ;
360+ int idx ;
361+
362+ public TreeNode BuildTree (int []inorder ,int []postorder ) {
363+ this .postorder = postorder ;
364+ inorderIdx = new Dictionary <int ,int >();
365+ for (int i = 0 ;i < inorder .Length ;i ++ ) {
366+ inorderIdx [inorder [i ]]= i ;
367+ }
368+ idx = postorder .Length - 1 ;
369+ return Dfs (0 ,inorder .Length - 1 );
370+ }
371+
372+ private TreeNode Dfs (int l ,int r ) {
373+ if (l > r )return null ;
374+
375+ TreeNode root = new TreeNode (postorder [idx -- ]);
376+ int i = inorderIdx [root .val ];
377+ root .right = Dfs (i + 1 ,r );
378+ root .left = Dfs (l ,i - 1 );
379+ return root ;
380+ }
381+ }
382+ ```
383+
304384:: tabs-end
305385
306386###Time & Space Complexity
@@ -468,9 +548,49 @@ class Solution {
468548}
469549```
470550
551+ ``` csharp
552+ /**
553+ * Definition for a binary tree node.
554+ * public class TreeNode {
555+ * public int val;
556+ * public TreeNode left;
557+ * public TreeNode right;
558+ * public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
559+ * this.val = val;
560+ * this.left = left;
561+ * this.right = right;
562+ * }
563+ * }
564+ */
565+ public class Solution {
566+ int postIdx ,inIdx ;
567+ int []inorder ,postorder ;
568+
569+ public TreeNode BuildTree (int []inorder ,int []postorder ) {
570+ this .inorder = inorder ;
571+ this .postorder = postorder ;
572+ postIdx = inIdx = postorder .Length - 1 ;
573+ return Dfs (int .MaxValue );
574+ }
575+
576+ private TreeNode Dfs (int limit ) {
577+ if (postIdx < 0 )return null ;
578+ if (inorder [inIdx ]== limit ) {
579+ inIdx -- ;
580+ return null ;
581+ }
582+
583+ TreeNode root = new TreeNode (postorder [postIdx -- ]);
584+ root .right = Dfs (root .val );
585+ root .left = Dfs (limit );
586+ return root ;
587+ }
588+ }
589+ ```
590+
471591:: tabs-end
472592
473593###Time & Space Complexity
474594
475595- Time complexity: $O(n)$
476- - Space complexity: $O(n)$ for recursion stack.
596+ - Space complexity: $O(n)$ for recursion stack.