feat: Add binary tree inorder traversal solution

Author: pragusga25

easy/binary-tree-inorder-traversal.test.ts

@@ -0,0 +1,67 @@
+// Problem: https://leetcode.com/problems/binary-tree-inorder-traversal/
+// Doc: https://leetcode.com/problems/binary-tree-inorder-traversal/solutions/5505544/inorder-traversal-of-binary-tree-iterative-approach/
+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;
+  }
+}
+
+function inorderTraversal(root: TreeNode | null): number[] {
+  if (!root) return [];
+  const stack: TreeNode[] = [];
+  const result: number[] = [];
+
+  while (stack.length > 0 || root) {
+    while (root) {
+      stack.push(root);
+      root = root.left;
+    }
+
+    const popped = stack.pop();
+    if (!popped) break;
+    result.push(popped.val);
+    root = popped.right;
+  }
+
+  return result;
+}
+
+describe('Binary Tree Inorder Traversal', () => {
+  it('#1 should return [1, 3, 2]', () => {
+    const root = new TreeNode(1, null, new TreeNode(2, new TreeNode(3)));
+    expect(inorderTraversal(root)).toStrictEqual([1, 3, 2]);
+  });
+
+  it('#2 should return []', () => {
+    expect(inorderTraversal(null)).toStrictEqual([]);
+  });
+
+  it('#3 should return [1]', () => {
+    const root = new TreeNode(1);
+    expect(inorderTraversal(root)).toStrictEqual([1]);
+  });
+
+  it('#4 should return [1, 2]', () => {
+    const root = new TreeNode(1, null, new TreeNode(2));
+    expect(inorderTraversal(root)).toStrictEqual([1, 2]);
+  });
+
+  it('#5 should return [2, 3, 1]', () => {
+    const root = new TreeNode(1, new TreeNode(3, new TreeNode(2)), null);
+    expect(inorderTraversal(root)).toStrictEqual([2, 3, 1]);
+  });
+
+  it('#6 should return [2, 4, 1, 5, 3]', () => {
+    const root = new TreeNode(
+      1,
+      new TreeNode(2, null, new TreeNode(4)),
+      new TreeNode(3, new TreeNode(5))
+    );
+    expect(inorderTraversal(root)).toStrictEqual([2, 4, 1, 5, 3]);
+  });
+});