add: BuildBinaryTree

Author: Jer3myYu

java/Trees/BuildBinaryTree.java

@@ -0,0 +1,50 @@
+import java.util.HashMap;
+import java.util.Map;
+
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class BuildBinaryTree {
+    int preorderIndex = 0;
+    Map<Integer, Integer> inorderIndexesMap = new HashMap<>();
+    
+    public TreeNode buildBinaryTree(int[] preorder, int[] inorder) {
+        // Populate the hash map with the inorder values and their indexes.
+        for (int i = 0; i < inorder.length; i++) {
+            inorderIndexesMap.put(inorder[i], i);
+        }
+        // Build the tree and return its root node.
+        return buildSubtree(0, inorder.length - 1, preorder, inorder);
+    }
+
+    private TreeNode buildSubtree(int left, int right, int[] preorder, int[] inorder) {
+        // Base case: if no elements are in this range, return None.
+        if (left > right) {
+            return null;
+        }
+        int val = preorder[preorderIndex];
+        // Set 'inorderIndex' to the index of the same value pointed at by
+        // 'preorderIndex'.
+        int inorderIndex = inorderIndexesMap.get(val);
+        TreeNode node = new TreeNode(val);
+        // Advance 'preorderIndex' so it points to the value of the next
+        // node to be created.
+        preorderIndex++;
+        // Build the left and right subtrees and connect them to the current
+        // node.
+        node.left = buildSubtree(left, inorderIndex - 1, preorder, inorder);
+        node.right = buildSubtree(inorderIndex + 1, right, preorder, inorder);
+        return node;
+    }
+}