899_Construct_BT_From_Pre_Post_Traversal.java

Author: vishwajeetmishra4

JAVA/899_Construct_BT_From_Pre_Post_Traversal.java

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+// Problem Number: 889
+
+// Construct Binary Tree From Preorder and PostOrder Traversal
+
+/**
+ * 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[] pre, int[] post) {
+    Map<Integer, Integer> postToIndex = new HashMap<>();
+
+    for (int i = 0; i < post.length; ++i)
+      postToIndex.put(post[i], i);
+
+    return build(pre, 0, pre.length - 1, post, 0, post.length - 1, postToIndex);
+  }
+
+  private TreeNode build(int[] pre, int preStart, int preEnd, int[] post, int postStart,
+                         int postEnd, Map<Integer, Integer> postToIndex) {
+    if (preStart > preEnd)
+      return null;
+    if (preStart == preEnd)
+      return new TreeNode(pre[preStart]);
+
+    final int rootVal = pre[preStart];
+    final int leftRootVal = pre[preStart + 1];
+    final int leftRootPostIndex = postToIndex.get(leftRootVal);
+    final int leftSize = leftRootPostIndex - postStart + 1;
+
+    TreeNode root = new TreeNode(rootVal);
+    root.left = build(pre, preStart + 1, preStart + leftSize, post, postStart, leftRootPostIndex,
+                      postToIndex);
+    root.right = build(pre, preStart + leftSize + 1, preEnd, post, leftRootPostIndex + 1,
+                       postEnd - 1, postToIndex);
+    return root;
+  }
+}