# Problem Statement

Given *n*, how many structurally unique **BST’s** (binary search trees) that store values 1…*n*?

For example,

Given *n* = 3, there are a total of 5 unique BST’s.

1 3 3 2 1 \ / / / \ \ 3 2 1 1 3 2 / / \ \ 2 1 2 3

Given *n*, how many structurally unique **BST’s** (binary search trees) that store values 1…*n*?

For example,

Given *n* = 3, there are a total of 5 unique BST’s.

1 3 3 2 1 \ / / / \ \ 3 2 1 1 3 2 / / \ \ 2 1 2 3

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Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example:

Given the below binary tree and `sum = 22`

,

5 / \ 4 8 / / \ 11 13 4 / \ \ 7 2 1

return true, as there exist a root-to-leaf path `5->4->11->2`

which sum is 22.

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

**Input:** (2 -> 4 -> 3) + (5 -> 6 -> 4)

**Output:** 7 -> 0 -> 8

Given a binary tree, flatten it to a linked list in-place.

For example,

Given

1 / \ 2 5 / \ \ 3 4 6

The flattened tree should look like:

1 \ 2 \ 3 \ 4 \ 5 \ 6

Given a linked list, remove the *n*^{th} node from the end of list and return its head

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