Huahua’s Tech Road

花花酱 LeetCode 2185. Counting Words With a Given Prefix

By zxi on February 26, 2022

You are given an array of strings words and a string pref.

Return the number of strings in words that contain pref as a prefix.

A prefix of a string s is any leading contiguous substring of s.

Example 1:

Input: words = ["pay","attention","practice","attend"], pref = "at"
Output: 2
Explanation: The 2 strings that contain "at" as a prefix are: "attention" and "attend".

Example 2:

Input: words = ["leetcode","win","loops","success"], pref = "code"
Output: 0
Explanation: There are no strings that contain "code" as a prefix.

Constraints:

1 <= words.length <= 100
1 <= words[i].length, pref.length <= 100
words[i] and pref consist of lowercase English letters.

Solution: Straight forward

We can use std::count_if and std::string::find.

Time complexity: O(n*l)
Space complexity: O(1)

C++

// Author: Huahua

class Solution {

public:

int prefixCount(vector<string>& words, string pref) {

return count_if(begin(words), end(words), [&](const string& w) {

return w.find(pref) == 0;

});

}

};

花花酱 LeetCode 2169. Count Operations to Obtain Zero

By zxi on February 13, 2022

You are given two non-negative integers num1 and num2.

In one operation, if num1 >= num2, you must subtract num2 from num1, otherwise subtract num1 from num2.

For example, if num1 = 5 and num2 = 4, subtract num2 from num1, thus obtaining num1 = 1 and num2 = 4. However, if num1 = 4 and num2 = 5, after one operation, num1 = 4 and num2 = 1.

Return the number of operations required to make either num1 = 0 or num2 = 0.

Example 1:

Input: num1 = 2, num2 = 3
Output: 3
Explanation: 
- Operation 1: num1 = 2, num2 = 3. Since num1 < num2, we subtract num1 from num2 and get num1 = 2, num2 = 3 - 2 = 1.
- Operation 2: num1 = 2, num2 = 1. Since num1 > num2, we subtract num2 from num1.
- Operation 3: num1 = 1, num2 = 1. Since num1 == num2, we subtract num2 from num1.
Now num1 = 0 and num2 = 1. Since num1 == 0, we do not need to perform any further operations.
So the total number of operations required is 3.

Example 2:

Input: num1 = 10, num2 = 10
Output: 1
Explanation: 
- Operation 1: num1 = 10, num2 = 10. Since num1 == num2, we subtract num2 from num1 and get num1 = 10 - 10 = 0.
Now num1 = 0 and num2 = 10. Since num1 == 0, we are done.
So the total number of operations required is 1.

Constraints:

0 <= num1, num2 <= 10⁵

Solution 1: Simulation

Time complexity: O(max(n,m) / min(n, m))
Space complexity: O(1)

No code

Solution 2: Simualtion + Math

For the case of 100, 3
100 – 3 = 97
97 – 3 = 94
…
4 – 3 = 1
Swap
3 – 1 = 2
2 – 1 = 1
1 – 1 = 0
It takes 36 steps.

We can do 100 / 3 to skip 33 steps
100 %= 3 = 1
3 / 1 = 3 to skip 3 steps
3 %= 1 = 0
total is 33 + 3 = 36.

Time complexity: O(logn) ?
Space complexity: O(1)

C++

// Author: Huahua

class Solution {

public:

int countOperations(int num1, int num2) {

int ans = 0;

while (num1 && num2) {

if (num1 < num2) swap(num1, num2);

ans += num1 / num2;

num1 %= num2;

}

return ans;

}

};

花花酱 LeetCode 2170. Minimum Operations to Make the Array Alternating

By zxi on February 13, 2022

You are given a 0-indexed array nums consisting of n positive integers.

The array nums is called alternating if:

nums[i - 2] == nums[i], where 2 <= i <= n - 1.
nums[i - 1] != nums[i], where 1 <= i <= n - 1.

In one operation, you can choose an index i and change nums[i] into any positive integer.

Return the minimum number of operations required to make the array alternating.

Example 1:

Input: nums = [3,1,3,2,4,3]
Output: 3
Explanation:
One way to make the array alternating is by converting it to [3,1,3,1,3,1].
The number of operations required in this case is 3.
It can be proven that it is not possible to make the array alternating in less than 3 operations.

Example 2:

Input: nums = [1,2,2,2,2]
Output: 2
Explanation:
One way to make the array alternating is by converting it to [1,2,1,2,1].
The number of operations required in this case is 2.
Note that the array cannot be converted to [2,2,2,2,2] because in this case nums[0] == nums[1] which violates the conditions of an alternating array.

Constraints:

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

Solution: Greedy

Count and sort the frequency of numbers at odd and even positions.

Case 1: top frequency numbers are different, change the rest of numbers to them respectively.
Case 2: top frequency numbers are the same, compare top 1 odd + top 2 even vs top 2 even + top 1 odd.

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

C++

// Author: Huahua

class Solution {

public:

int minimumOperations(vector<int>& nums) {

const int n = nums.size();

if (n == 1) return 0;

unordered_map<int, int> odd, even;

for (int i = 0; i < n; ++i)

((i & 1 ? odd : even)[nums[i]])++;

vector<pair<int, int>> o, e;

for (const auto [k, v] : odd)

o.emplace_back(v, k);

for (const auto [k, v] : even)

e.emplace_back(v, k);

sort(rbegin(o), rend(o));

sort(rbegin(e), rend(e));

if (o[0].second != e[0].second)

return n - o[0].first - e[0].first;

int mo = o[0].first + (e.size() > 1 ? e[1].first : 0);

int me = e[0].first + (o.size() > 1 ? o[1].first : 0);

return n - max(mo, me);

}

};

花花酱 LeetCode 2157. Groups of Strings

By zxi on February 6, 2022

You are given a 0-indexed array of strings words. Each string consists of lowercase English letters only. No letter occurs more than once in any string of words.

Two strings s1 and s2 are said to be connected if the set of letters of s2 can be obtained from the set of letters of s1 by any one of the following operations:

Adding exactly one letter to the set of the letters of s1.
Deleting exactly one letter from the set of the letters of s1.
Replacing exactly one letter from the set of the letters of s1 with any letter, including itself.

The array words can be divided into one or more non-intersecting groups. A string belongs to a group if any one of the following is true:

It is connected to at least one other string of the group.
It is the only string present in the group.

Note that the strings in words should be grouped in such a manner that a string belonging to a group cannot be connected to a string present in any other group. It can be proved that such an arrangement is always unique.

Return an array ans of size 2 where:

ans[0] is the total number of groups words can be divided into, and
ans[1] is the size of the largest group.

Example 1:

Input: words = ["a","b","ab","cde"]
Output: [2,3]
Explanation:
- words[0] can be used to obtain words[1] (by replacing 'a' with 'b'), and words[2] (by adding 'b'). So words[0] is connected to words[1] and words[2].
- words[1] can be used to obtain words[0] (by replacing 'b' with 'a'), and words[2] (by adding 'a'). So words[1] is connected to words[0] and words[2].
- words[2] can be used to obtain words[0] (by deleting 'b'), and words[1] (by deleting 'a'). So words[2] is connected to words[0] and words[1].
- words[3] is not connected to any string in words.
Thus, words can be divided into 2 groups ["a","b","ab"] and ["cde"]. The size of the largest group is 3.

Example 2:

Input: words = ["a","ab","abc"]
Output: [1,3]
Explanation:
- words[0] is connected to words[1].
- words[1] is connected to words[0] and words[2].
- words[2] is connected to words[1].
Since all strings are connected to each other, they should be grouped together.
Thus, the size of the largest group is 3.

Constraints:

1 <= words.length <= 2 * 10⁴
1 <= words[i].length <= 26
words[i] consists of lowercase English letters only.
No letter occurs more than once in words[i].

Solution: Bitmask + DFS

Use a bitmask to represent a string. Use dfs to find connected components.

Time complexity: O(n*26²)
Space complexity: O(n)

C++

// Author: Huahua

class Solution {

public:

vector<int> groupStrings(vector<string>& words) {

unordered_map<int, int> m;

for (const string& w : words)

++m[accumulate(begin(w), end(w), 0, [](int k, char c){ return k | (1 << (c - 'a')); })];

function<int(int)> dfs = [&](int mask) {

auto it = m.find(mask);

if (it == end(m)) return 0;

int ans = it->second;

m.erase(it);

for (int i = 0; i < 26; ++i) {

ans += dfs(mask ^ (1 << i));

for (int j = i + 1; j < 26; ++j)

if ((mask >> i & 1) != (mask >> j & 1))

ans += dfs(mask ^ (1 << i) ^ (1 << j));

}

return ans;

};

int size = 0;

int groups = 0;

while (!m.empty()) {

size = max(size, dfs(begin(m)->first));

++groups;

}

return {groups, size};

}

};

花花酱 LeetCode 2156. Find Substring With Given Hash Value

By zxi on February 5, 2022

The hash of a 0-indexed string s of length k, given integers p and m, is computed using the following function:

hash(s, p, m) = (val(s[0]) * p⁰ + val(s[1]) * p¹ + ... + val(s[k-1]) * p^k-1) mod m.

Where val(s[i]) represents the index of s[i] in the alphabet from val('a') = 1 to val('z') = 26.

You are given a string s and the integers power, modulo, k, and hashValue. Return sub, the first substring of s of length k such that hash(sub, power, modulo) == hashValue.

The test cases will be generated such that an answer always exists.

A substring is a contiguous non-empty sequence of characters within a string.

Example 1:

Input: s = "leetcode", power = 7, modulo = 20, k = 2, hashValue = 0
Output: "ee"
Explanation: The hash of "ee" can be computed to be hash("ee", 7, 20) = (5 * 1 + 5 * 7) mod 20 = 40 mod 20 = 0. 
"ee" is the first substring of length 2 with hashValue 0. Hence, we return "ee".

Example 2:

Input: s = "fbxzaad", power = 31, modulo = 100, k = 3, hashValue = 32
Output: "fbx"
Explanation: The hash of "fbx" can be computed to be hash("fbx", 31, 100) = (6 * 1 + 2 * 31 + 24 * 31²) mod 100 = 23132 mod 100 = 32. 
The hash of "bxz" can be computed to be hash("bxz", 31, 100) = (2 * 1 + 24 * 31 + 26 * 31²) mod 100 = 25732 mod 100 = 32. 
"fbx" is the first substring of length 3 with hashValue 32. Hence, we return "fbx".
Note that "bxz" also has a hash of 32 but it appears later than "fbx".

Constraints:

1 <= k <= s.length <= 2 * 10⁴
1 <= power, modulo <= 10⁹
0 <= hashValue < modulo
s consists of lowercase English letters only.
The test cases are generated such that an answer always exists.

Solution: Sliding window

hash = (((hash – (s[i+k] * p^k-1) % mod + mod) * p) + (s[i] * p⁰)) % mod

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

C++

// Author: Huahua

class Solution {

public:

string subStrHash(string s, int power, int modulo, int k, int hashValue) {

const int n = s.length();

long p = 1;

for (int i = 1; i < k; ++i)

p = (p * power) % modulo;

long ans = 0;

for (long i = n - 1, cur = 0; i >= 0; --i) {

if (i + k < n)

cur = ((cur - (s[i + k] - 'a' + 1) * p) % modulo + modulo) % modulo;

cur = (cur * power + (s[i] - 'a' + 1)) % modulo;

if (i + k <= n && cur == hashValue)

ans = i;

}

return s.substr(ans, k);

}

};

Huahua's Tech Road

花花酱 LeetCode 2185. Counting Words With a Given Prefix

Solution: Straight forward

C++

花花酱 LeetCode 2169. Count Operations to Obtain Zero

Solution 1: Simulation

Solution 2: Simualtion + Math

C++

花花酱 LeetCode 2170. Minimum Operations to Make the Array Alternating

Solution: Greedy

C++

花花酱 LeetCode 2157. Groups of Strings

Solution: Bitmask + DFS

C++

花花酱 LeetCode 2156. Find Substring With Given Hash Value

Solution: Sliding window

C++