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Java-based LeetCode algorithm problem solutions, regularly updated

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1643\. Kth Smallest Instructions

Hard

Bob is standing at cell `(0, 0)`, and he wants to reach `destination`: `(row, column)`. He can only travel **right** and **down**. You are going to help Bob by providing **instructions** for him to reach `destination`.

The **instructions** are represented as a string, where each character is either:

*   `'H'`, meaning move horizontally (go **right**), or
*   `'V'`, meaning move vertically (go **down**).

Multiple **instructions** will lead Bob to `destination`. For example, if `destination` is `(2, 3)`, both `"HHHVV"` and `"HVHVH"` are valid **instructions**.

However, Bob is very picky. Bob has a lucky number `k`, and he wants the k^th **lexicographically smallest instructions** that will lead him to `destination`. `k` is **1-indexed**.

Given an integer array `destination` and an integer `k`, return _the_ k^th _**lexicographically smallest instructions** that will take Bob to_ `destination`.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/10/12/ex1.png)

**Input:** destination = [2,3], k = 1

**Output:** "HHHVV"

**Explanation:** All the instructions that reach (2, 3) in lexicographic order are as follows: ["HHHVV", "HHVHV", "HHVVH", "HVHHV", "HVHVH", "HVVHH", "VHHHV", "VHHVH", "VHVHH", "VVHHH"].

**Example 2:**

**![](https://assets.leetcode.com/uploads/2020/10/12/ex2.png)**

**Input:** destination = [2,3], k = 2

**Output:** "HHVHV"

**Example 3:**

**![](https://assets.leetcode.com/uploads/2020/10/12/ex3.png)**

**Input:** destination = [2,3], k = 3

**Output:** "HHVVH"

**Constraints:**

*   `destination.length == 2`
*   `1 <= row, column <= 15`
*   `1 <= k <= nCr(row + column, row)`, where `nCr(a, b)` denotes `a` choose `b`.