Create Construct Binary Tree from Preorder and Postorder Traversal.java

varunu28 · web-flow · commit c25f01b2f86a · 2025-02-23T10:38:59.000-08:00
diff --git a/Medium/Construct Binary Tree from Preorder and Postorder Traversal.java b/Medium/Construct Binary Tree from Preorder and Postorder Traversal.java
@@ -0,0 +1,40 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    public TreeNode constructFromPrePost(int[] preorder, int[] postorder) {
+        int n = preorder.length;
+        Map<Integer, Integer> postorderIndex = new HashMap<>();
+        for (int i = 0; i < n; i++) {
+            postorderIndex.put(postorder[i], i);
+        }
+        return constructTree(0, n - 1, 0, preorder, postorderIndex);
+    }
+
+    private TreeNode constructTree(int preStart, int preEnd, int postStart, int[] preorder, Map<Integer, Integer> postorderIndex) {
+        if (preStart > preEnd) {
+            return null;
+        }
+        if (preStart == preEnd) {
+            return new TreeNode(preorder[preStart]);
+        }
+        int leftRoot = preorder[preStart + 1];
+        int leftCount = postorderIndex.get(leftRoot) - postStart + 1;
+        TreeNode root = new TreeNode(preorder[preStart]);
+        root.left = constructTree(preStart + 1, preStart + leftCount, postStart, preorder, postorderIndex);
+        root.right = constructTree(preStart + leftCount + 1, preEnd, postStart + leftCount, preorder, postorderIndex);
+        return root;
+    }
+}
