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Posts tagged as “medium”

花花酱 LeetCode 2560. House Robber IV

There are several consecutive houses along a street, each of which has some money inside. There is also a robber, who wants to steal money from the homes, but he refuses to steal from adjacent homes.

The capability of the robber is the maximum amount of money he steals from one house of all the houses he robbed.

You are given an integer array nums representing how much money is stashed in each house. More formally, the ith house from the left has nums[i] dollars.

You are also given an integer k, representing the minimum number of houses the robber will steal from. It is always possible to steal at least k houses.

Return the minimum capability of the robber out of all the possible ways to steal at least k houses.

Example 1:

Input: nums = [2,3,5,9], k = 2
Output: 5
Explanation: 
There are three ways to rob at least 2 houses:
- Rob the houses at indices 0 and 2. Capability is max(nums[0], nums[2]) = 5.
- Rob the houses at indices 0 and 3. Capability is max(nums[0], nums[3]) = 9.
- Rob the houses at indices 1 and 3. Capability is max(nums[1], nums[3]) = 9.
Therefore, we return min(5, 9, 9) = 5.

Example 2:

Input: nums = [2,7,9,3,1], k = 2
Output: 2
Explanation: There are 7 ways to rob the houses. The way which leads to minimum capability is to rob the house at index 0 and 4. Return max(nums[0], nums[4]) = 2.

Constraints:

  • 1 <= nums.length <= 105
  • 1 <= nums[i] <= 109
  • 1 <= k <= (nums.length + 1)/2

Solution 1: Binary Search + DP

It’s easy to see that higher capability means more houses we can rob. Thus this can be formulate as a binary search algorithm e.g. find the minimum C s.t. we can rob at least k houses.

Then we can use dp(i) to calculate maximum houses we can rob if starting from the i’th house.
dp(i) = max(1 + dp(i + 2) if nums[i] <= C else 0, dp(i + 1))

Time complexity: O(n log m)
Space complexity: O(n)

C++

Solution 2: Binary Search + Greedy

From: dp(i) = max(1 + dp(i + 2) if nums[i] <= C else 0, dp(i + 1)) we can see that if we can pick the i-th one, it will be the same or better if we skip and start from dp(i + 1). Thus we can convert this from DP to greedy. As long as we can pick the current one, we pick it first.

Time complexity: O(n log m)
Space complexity: O(1)

C++

花花酱 LeetCode 1061. Lexicographically Smallest Equivalent String

You are given two strings of the same length s1 and s2 and a string baseStr.

We say s1[i] and s2[i] are equivalent characters.

  • For example, if s1 = "abc" and s2 = "cde", then we have 'a' == 'c''b' == 'd', and 'c' == 'e'.

Equivalent characters follow the usual rules of any equivalence relation:

  • Reflexivity: 'a' == 'a'.
  • Symmetry: 'a' == 'b' implies 'b' == 'a'.
  • Transitivity: 'a' == 'b' and 'b' == 'c' implies 'a' == 'c'.

For example, given the equivalency information from s1 = "abc" and s2 = "cde""acd" and "aab" are equivalent strings of baseStr = "eed", and "aab" is the lexicographically smallest equivalent string of baseStr.

Return the lexicographically smallest equivalent string of baseStr by using the equivalency information from s1 and s2.

Example 1:

Input: s1 = "parker", s2 = "morris", baseStr = "parser"
Output: "makkek"
Explanation: Based on the equivalency information in s1 and s2, we can group their characters as [m,p], [a,o], [k,r,s], [e,i].
The characters in each group are equivalent and sorted in lexicographical order.
So the answer is "makkek".

Example 2:

Input: s1 = "hello", s2 = "world", baseStr = "hold"
Output: "hdld"
Explanation: Based on the equivalency information in s1 and s2, we can group their characters as [h,w], [d,e,o], [l,r].
So only the second letter 'o' in baseStr is changed to 'd', the answer is "hdld".

Example 3:

Constraints:

  • 1 <= s1.length, s2.length, baseStr <= 1000
  • s1.length == s2.length
  • s1s2, and baseStr consist of lowercase English letters.

Solution: Union Find

Time complexity: O(n + m)
Space complexity: O(1)

C++

花花酱 LeetCode 2477. Minimum Fuel Cost to Report to the Capital

There is a tree (i.e., a connected, undirected graph with no cycles) structure country network consisting of n cities numbered from 0 to n - 1 and exactly n - 1 roads. The capital city is city 0. You are given a 2D integer array roads where roads[i] = [ai, bi] denotes that there exists a bidirectional road connecting cities ai and bi.

There is a meeting for the representatives of each city. The meeting is in the capital city.

There is a car in each city. You are given an integer seats that indicates the number of seats in each car.

A representative can use the car in their city to travel or change the car and ride with another representative. The cost of traveling between two cities is one liter of fuel.

Return the minimum number of liters of fuel to reach the capital city.

Example 1:

Input: roads = [[0,1],[0,2],[0,3]], seats = 5
Output: 3
Explanation: 
- Representative1 goes directly to the capital with 1 liter of fuel.
- Representative2 goes directly to the capital with 1 liter of fuel.
- Representative3 goes directly to the capital with 1 liter of fuel.
It costs 3 liters of fuel at minimum. 
It can be proven that 3 is the minimum number of liters of fuel needed.

Example 2:

Input: roads = [[3,1],[3,2],[1,0],[0,4],[0,5],[4,6]], seats = 2
Output: 7
Explanation: 
- Representative2 goes directly to city 3 with 1 liter of fuel.
- Representative2 and representative3 go together to city 1 with 1 liter of fuel.
- Representative2 and representative3 go together to the capital with 1 liter of fuel.
- Representative1 goes directly to the capital with 1 liter of fuel.
- Representative5 goes directly to the capital with 1 liter of fuel.
- Representative6 goes directly to city 4 with 1 liter of fuel.
- Representative4 and representative6 go together to the capital with 1 liter of fuel.
It costs 7 liters of fuel at minimum. 
It can be proven that 7 is the minimum number of liters of fuel needed.

Example 3:

Input: roads = [], seats = 1
Output: 0
Explanation: No representatives need to travel to the capital city.

Constraints:

  • 1 <= n <= 105
  • roads.length == n - 1
  • roads[i].length == 2
  • 0 <= ai, bi < n
  • ai != bi
  • roads represents a valid tree.
  • 1 <= seats <= 105

Solution: Greedy + DFS

To reach the minimum cost, we must share cars if possible, say X reps from children nodes to an intermediate node u on the way towards capital 0. Then they all changes cars at node u, and we need (X + 1) // seats cars/fuel from u to 0.

We use DFS to count # of reps at each node u while accumulating the total cost.

Time complexity: O(n)
Space complexity: O(n)

C++

花花酱 LeetCode 2476. Closest Nodes Queries in a Binary Search Tree

You are given the root of a binary search tree and an array queries of size n consisting of positive integers.

Find a 2D array answer of size n where answer[i] = [mini, maxi]:

  • mini is the largest value in the tree that is smaller than or equal to queries[i]. If a such value does not exist, add -1 instead.
  • maxi is the smallest value in the tree that is greater than or equal to queries[i]. If a such value does not exist, add -1 instead.

Return the array answer.

Example 1:

Input: root = [6,2,13,1,4,9,15,null,null,null,null,null,null,14], queries = [2,5,16]
Output: [[2,2],[4,6],[15,-1]]
Explanation: We answer the queries in the following way:
- The largest number that is smaller or equal than 2 in the tree is 2, and the smallest number that is greater or equal than 2 is still 2. So the answer for the first query is [2,2].
- The largest number that is smaller or equal than 5 in the tree is 4, and the smallest number that is greater or equal than 5 is 6. So the answer for the second query is [4,6].
- The largest number that is smaller or equal than 16 in the tree is 15, and the smallest number that is greater or equal than 16 does not exist. So the answer for the third query is [15,-1].

Example 2:

Input: root = [4,null,9], queries = [3]
Output: [[-1,4]]
Explanation: The largest number that is smaller or equal to 3 in the tree does not exist, and the smallest number that is greater or equal to 3 is 4. So the answer for the query is [-1,4].

Constraints:

  • The number of nodes in the tree is in the range [2, 105].
  • 1 <= Node.val <= 106
  • n == queries.length
  • 1 <= n <= 105
  • 1 <= queries[i] <= 106

Solution: Convert to sorted array

Since we don’t know whether the tree is balanced or not, the safest and easiest way is to convert the tree into a sorted array using inorder traversal. Or just any traversal and sort the array later on.

Once we have a sorted array, we can use lower_bound / upper_bound to query.

Time complexity: O(qlogn)
Space complexity: O(n)

C++

One binary search per query.

C++

花花酱 LeetCode 2452. Words Within Two Edits of Dictionary

You are given two string arrays, queries and dictionary. All words in each array comprise of lowercase English letters and have the same length.

In one edit you can take a word from queries, and change any letter in it to any other letter. Find all words from queries that, after a maximum of two edits, equal some word from dictionary.

Return a list of all words from queriesthat match with some word from dictionary after a maximum of two edits. Return the words in the same order they appear in queries.

Example 1:

Input: queries = ["word","note","ants","wood"], dictionary = ["wood","joke","moat"]
Output: ["word","note","wood"]
Explanation:
- Changing the 'r' in "word" to 'o' allows it to equal the dictionary word "wood".
- Changing the 'n' to 'j' and the 't' to 'k' in "note" changes it to "joke".
- It would take more than 2 edits for "ants" to equal a dictionary word.
- "wood" can remain unchanged (0 edits) and match the corresponding dictionary word.
Thus, we return ["word","note","wood"].

Example 2:

Input: queries = ["yes"], dictionary = ["not"]
Output: []
Explanation:
Applying any two edits to "yes" cannot make it equal to "not". Thus, we return an empty array.

Constraints:

  • 1 <= queries.length, dictionary.length <= 100
  • n == queries[i].length == dictionary[j].length
  • 1 <= n <= 100
  • All queries[i] and dictionary[j] are composed of lowercase English letters.

Solution: Hamming distance + Brute Force

For each query word q, check the hamming distance between it and all words in the dictionary.

Time complexity: O(|q|*|d|*n)
Space complexity: O(1)

C++