Binary Tree Boundary Traversal

uber Given a binary tree, write a function to return the values of the boundary nodes. The boundary should start from the left-bottom, pass through the root, and end at the right-bottom. The time complexity should be O(n), where n is the number of nodes. Provide the structure and constructor for the binary tree. Example Input: 1 / \ 2 3 \ \ 5 4 Output: [2, 1, 3, 4] Example Input 1 2 3 -1 5 -1 4