Add construct-binary-tree-from-preorder-and-inorder-traversal solution in TS

Author: Jeehay28

construct-binary-tree-from-preorder-and-inorder-traversal/Jeehay28.ts

@@ -0,0 +1,44 @@
+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;
+  }
+}
+
+// TC: O(n)
+// SC: O(n)
+function buildTree(preorder: number[], inorder: number[]): TreeNode | null {
+  const indices = {};
+  //     indices = {
+  //   9: 0,
+  //   3: 1,
+  //   15: 2,
+  //   20: 3,
+  //   7: 4
+  // }
+
+  for (let i = 0; i < inorder.length; i++) {
+    const num = inorder[i];
+    indices[num] = i;
+  }
+
+  let preIndex = 0;
+
+  const dfs = (start: number, end: number) => {
+    if (start > end) return null;
+    const val = preorder[preIndex];
+    preIndex++;
+    const mid = indices[val];
+
+    const left = dfs(start, mid - 1);
+    const right = dfs(mid + 1, end);
+
+    return new TreeNode(val, left, right);
+  };
+
+  return dfs(0, inorder.length - 1);
+}