construct binary tree from preorder and inorder traversal solution

Author: hyer0705

construct-binary-tree-from-preorder-and-inorder-traversal/hyer0705.ts

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+/**
+ * Definition for a binary tree node.
+ * class TreeNode {
+ *     val: number
+ *     left: TreeNode | null
+ *     right: TreeNode | null
+ *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+ *         this.val = (val===undefined ? 0 : val)
+ *         this.left = (left===undefined ? null : left)
+ *         this.right = (right===undefined ? null : right)
+ *     }
+ * }
+ */
+
+// 37ms
+// Time Complexity: O(n^2), n: 노드의 수
+// Space Complexity: O(h), h: 트리의 높이
+function buildTree(preorder: number[], inorder: number[]): TreeNode | null {
+  if (preorder.length === 0 || inorder.length === 0) return null;
+
+  const root = preorder[0];
+
+  const rootIdx = inorder.findIndex((el) => el === root);
+
+  const leftInorder = inorder.slice(0, rootIdx);
+  const leftPreorder = preorder.slice(1, leftInorder.length + 1);
+
+  const rightInorder = inorder.slice(rootIdx + 1);
+  const rightPreorder = preorder.slice(leftInorder.length + 1);
+
+  const rootNode = new TreeNode(root);
+
+  rootNode.left = buildTree(leftPreorder, leftInorder);
+  rootNode.right = buildTree(rightPreorder, rightInorder);
+
+  return rootNode;
+}
+
+// 3ms
+// Time Complexity: O(n), n: 노드의 수
+// Space Complexity: O(n), n: 노드의 수
+function buildTree(preorder: number[], inorder: number[]): TreeNode | null {
+  if (preorder.length === 0 || inorder.length === 0) return null;
+
+  const indexMap = new Map<number, number>();
+  inorder.forEach((node, idx) => indexMap.set(node, idx));
+
+  const build = (preStart: number, preEnd: number, inStart: number, inEnd: number): TreeNode | null => {
+    if (preStart > preEnd || inStart > inEnd) return null;
+
+    const rootVal = preorder[preStart];
+    const rootIdx = indexMap.get(rootVal);
+
+    const leftSize = rootIdx - inStart;
+
+    const root = new TreeNode(rootVal);
+    root.left = build(preStart + 1, preStart + leftSize, inStart, rootIdx - 1);
+    root.right = build(preStart + leftSize + 1, preEnd, rootIdx + 1, inEnd);
+
+    return root;
+  };
+
+  return build(0, preorder.length - 1, 0, inorder.length - 1);
+}