Posts published in “Hashtable”

花花酱 LeetCode 2190. Most Frequent Number Following Key In an Array

By zxi on March 11, 2022

You are given a 0-indexed integer array nums. You are also given an integer key, which is present in nums.

For every unique integer target in nums, count the number of times target immediately follows an occurrence of key in nums. In other words, count the number of indices i such that:

0 <= i <= nums.length - 2,
nums[i] == key and,
nums[i + 1] == target.

Return the target with the maximum count. The test cases will be generated such that the target with maximum count is unique.

Example 1:

Input: nums = [1,100,200,1,100], key = 1
Output: 100
Explanation: For target = 100, there are 2 occurrences at indices 1 and 4 which follow an occurrence of key.
No other integers follow an occurrence of key, so we return 100.

Example 2:

Input: nums = [2,2,2,2,3], key = 2
Output: 2
Explanation: For target = 2, there are 3 occurrences at indices 1, 2, and 3 which follow an occurrence of key.
For target = 3, there is only one occurrence at index 4 which follows an occurrence of key.
target = 2 has the maximum number of occurrences following an occurrence of key, so we return 2.

Constraints:

2 <= nums.length <= 1000
1 <= nums[i] <= 1000
The test cases will be generated such that the answer is unique.

Solution: Hashtable

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

C++

// Author: Huahua

class Solution {

public:

int mostFrequent(vector<int>& nums, int key) {

const int n = nums.size();

unordered_map<int, int> m;

int count = 0;

int ans = 0;

for (int i = 1; i < n; ++i) {

if (nums[i - 1] == key && ++m[nums[i]] > count) {

count = m[nums[i]];

ans = nums[i];

}

return ans;

}

};

花花酱 LeetCode 2186. Minimum Number of Steps to Make Two Strings Anagram II

By zxi on February 26, 2022

You are given two strings s and t. In one step, you can append any character to either s or t.

Return the minimum number of steps to make s and t anagrams of each other.

An anagram of a string is a string that contains the same characters with a different (or the same) ordering.

Example 1:

Input: s = "leetcode", t = "coats"
Output: 7
Explanation: 
- In 2 steps, we can append the letters in "as" onto s = "leetcode", forming s = "leetcodeas".
- In 5 steps, we can append the letters in "leede" onto t = "coats", forming t = "coatsleede".
"leetcodeas" and "coatsleede" are now anagrams of each other.
We used a total of 2 + 5 = 7 steps.
It can be shown that there is no way to make them anagrams of each other with less than 7 steps.

Example 2:

Input: s = "night", t = "thing"
Output: 0
Explanation: The given strings are already anagrams of each other. Thus, we do not need any further steps.

Constraints:

1 <= s.length, t.length <= 2 * 10⁵
s and t consist of lowercase English letters.

Solution: Hashtable

Record the frequency difference of each letter.

Ans = sum(diff)

Time complexity: O(m + n)
Space complexity: O(26)

C++

// Author: Huahua

class Solution {

public:

int minSteps(string s, string t) {

vector<int> diff(26);

for (char c : s) ++diff[c - 'a'];

for (char c : t) --diff[c - 'a'];

int ans = 0;

for (int d : diff)

ans += abs(d);

return ans;

}

};

花花酱 LeetCode 2154. Keep Multiplying Found Values by Two

By zxi on February 4, 2022

You are given an array of integers nums. You are also given an integer original which is the first number that needs to be searched for in nums.

You then do the following steps:

If original is found in nums, multiply it by two (i.e., set original = 2 * original).
Otherwise, stop the process.
Repeat this process with the new number as long as you keep finding the number.

Return the final value of original.

Example 1:

Input: nums = [5,3,6,1,12], original = 3
Output: 24
Explanation: 
- 3 is found in nums. 3 is multiplied by 2 to obtain 6.
- 6 is found in nums. 6 is multiplied by 2 to obtain 12.
- 12 is found in nums. 12 is multiplied by 2 to obtain 24.
- 24 is not found in nums. Thus, 24 is returned.

Example 2:

Input: nums = [2,7,9], original = 4
Output: 4
Explanation:
- 4 is not found in nums. Thus, 4 is returned.

Constraints:

1 <= nums.length <= 1000
1 <= nums[i], original <= 1000

Solution: Hashset

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

C++

// Author: Huahua

class Solution {

public:

int findFinalValue(vector<int>& nums, int original) {

unordered_set<int> s(begin(nums), end(nums));

while (true) {

if (!s.count(original)) break;

original *= 2;

}

return original;

}

};

花花酱 LeetCode 2150. Find All Lonely Numbers in the Array

By zxi on February 4, 2022

You are given an integer array nums. A number x is lonely when it appears only once, and no adjacent numbers (i.e. x + 1 and x - 1) appear in the array.

Return all lonely numbers in nums. You may return the answer in any order.

Example 1:

Input: nums = [10,6,5,8]
Output: [10,8]
Explanation: 
- 10 is a lonely number since it appears exactly once and 9 and 11 does not appear in nums.
- 8 is a lonely number since it appears exactly once and 7 and 9 does not appear in nums.
- 5 is not a lonely number since 6 appears in nums and vice versa.
Hence, the lonely numbers in nums are [10, 8].
Note that [8, 10] may also be returned.

Example 2:

Input: nums = [1,3,5,3]
Output: [1,5]
Explanation: 
- 1 is a lonely number since it appears exactly once and 0 and 2 does not appear in nums.
- 5 is a lonely number since it appears exactly once and 4 and 6 does not appear in nums.
- 3 is not a lonely number since it appears twice.
Hence, the lonely numbers in nums are [1, 5].
Note that [5, 1] may also be returned.

Constraints:

1 <= nums.length <= 10⁵
0 <= nums[i] <= 10⁶

Solution: Counter

Computer the frequency of each number in the array, for a given number x with freq = 1, check freq of (x – 1) and (x + 1), if both of them are zero then x is lonely.

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

C++

// Author: Huahua

class Solution {

public:

vector<int> findLonely(vector<int>& nums) {

unordered_map<int, int> m;

for (int x : nums)

++m[x];

vector<int> ans;

for (const auto [x, c] : m)

if (c == 1 && !m.count(x + 1) && !m.count(x - 1))

ans.push_back(x);

return ans;

}

};

花花酱 LeetCode 2135. Count Words Obtained After Adding a Letter

By zxi on January 17, 2022

You are given two 0-indexed arrays of strings startWords and targetWords. Each string consists of lowercase English letters only.

For each string in targetWords, check if it is possible to choose a string from startWords and perform a conversion operation on it to be equal to that from targetWords.

The conversion operation is described in the following two steps:

Append any lowercase letter that is not present in the string to its end.
- For example, if the string is "abc", the letters 'd', 'e', or 'y' can be added to it, but not 'a'. If 'd' is added, the resulting string will be "abcd".
Rearrange the letters of the new string in any arbitrary order.
- For example, "abcd" can be rearranged to "acbd", "bacd", "cbda", and so on. Note that it can also be rearranged to "abcd" itself.

Return the number of strings in targetWords that can be obtained by performing the operations on any string of startWords.

Note that you will only be verifying if the string in targetWords can be obtained from a string in startWords by performing the operations. The strings in startWords do not actually change during this process.

Example 1:

Input: startWords = ["ant","act","tack"], targetWords = ["tack","act","acti"]
Output: 2
Explanation:
- In order to form targetWords[0] = "tack", we use startWords[1] = "act", append 'k' to it, and rearrange "actk" to "tack".
- There is no string in startWords that can be used to obtain targetWords[1] = "act".
  Note that "act" does exist in startWords, but we must append one letter to the string before rearranging it.
- In order to form targetWords[2] = "acti", we use startWords[1] = "act", append 'i' to it, and rearrange "acti" to "acti" itself.

Example 2:

Input: startWords = ["ab","a"], targetWords = ["abc","abcd"]
Output: 1
Explanation:
- In order to form targetWords[0] = "abc", we use startWords[0] = "ab", add 'c' to it, and rearrange it to "abc".
- There is no string in startWords that can be used to obtain targetWords[1] = "abcd".

Constraints:

1 <= startWords.length, targetWords.length <= 5 * 10⁴
1 <= startWords[i].length, targetWords[j].length <= 26
Each string of startWords and targetWords consists of lowercase English letters only.
No letter occurs more than once in any string of startWords or targetWords.

Solution: Bitmask w/ Hashtable

Since there is no duplicate letters in each word, we can use a bitmask to represent a word.

Step 1: For each word in startWords, we obtain its bitmask and insert it into a hashtable.
Step 2: For each word in targetWords, enumerate it’s letter and unset 1 bit (skip one letter) and see whether it’s in the hashtable or not.

E.g. for target word “abc”, its bitmask is 0…0111, and we test whether “ab” or “ac” or “bc” in the hashtable or not.

Time complexity: O(n * 26^2)
Space complexity: O(n * 26)

C++

// Author: Huahua

class Solution {

public:

int wordCount(vector<string>& startWords, vector<string>& targetWords) {

unordered_set<int> s;

auto getKey = [](const string& s) {

return accumulate(begin(s), end(s), 0, [](int key, char c) { return key | (1 << (c - 'a')); });

};

for (const string& w : startWords)

s.insert(getKey(w));

return accumulate(begin(targetWords), end(targetWords), 0, [&](int ans, const string& w) {

const int key = getKey(w);

return ans + any_of(begin(w), end(w), [&](char c) { return s.count(key ^ (1 << (c - 'a'))); });

});

}

};