Today I have done Count Primes on leetcode and leetcode's October LeetCoding Challenge with cpp.

Count Primes

Description

Count the number of prime numbers less than a non-negative number, n.

Example 1:

Input: n = 10
Output: 4
Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.

Example 2:

Input: n = 0
Output: 0

Example 3:

Input: n = 1
Output: 0

Constraints:

• 0 <= n <= 5 * 106

Solution

sieve of Eratosthenes

class Solution {
public:
  int countPrimes(int n) {
    if(n < 3) return 0;
    int ans = 0;
    vector<bool> isPrime(n, true);
    for(int i = 2; i*i < n; ++i) {
      if(!isPrime[i]) continue;
      for(int j = i*i; j < n; j += i) {
        isPrime[j] = false;
      }
    }
    for(int i = 2; i < n; ++i) {
      ans += isPrime[i];
    }
    return ans;
  }
};

October LeetCoding Challenge 28

Description

Summary Ranges

You are given a sorted unique integer array nums.

Return the smallest sorted list of ranges that cover all the numbers in the array exactly. That is, each element of nums is covered by exactly one of the ranges, and there is no integer x such that x is in one of the ranges but not in nums.

Each range [a,b] in the list should be output as:

• "a->b" if a != b
• "a" if a == b

Example 1:

Input: nums = [0,1,2,4,5,7]
Output: ["0->2","4->5","7"]
Explanation: The ranges are:
[0,2] --> "0->2"
[4,5] --> "4->5"
[7,7] --> "7"

Example 2:

Input: nums = [0,2,3,4,6,8,9]
Output: ["0","2->4","6","8->9"]
Explanation: The ranges are:
[0,0] --> "0"
[2,4] --> "2->4"
[6,6] --> "6"
[8,9] --> "8->9"

Example 3:

Input: nums = []
Output: []

Example 4:

Input: nums = [-1]
Output: ["-1"]

Example 5:

Input: nums = [0]
Output: ["0"]

Constraints:

• 0 <= nums.length <= 20
• -231 <= nums[i] <= 231 - 1
• All the values of nums are unique.

Solution

nothing to say

class Solution {
public:
  vector<string> summaryRanges(vector<int>& nums) {
    vector<string> ans;
    if(nums.empty()) return ans;
    int begin = nums[0], end = nums[0];
    for(int i = 1; i < nums.size(); ++i) {
      if(nums[i]-1 != end) {
        string tmp = to_string(begin);
        if(end != begin) tmp += "->"+to_string(end);
        ans.push_back(tmp);
        begin = nums[i];
        end = nums[i];
      } else {
        end = nums[i];
      }
    }
    string tmp = to_string(begin);
    if(end != begin) tmp += "->"+to_string(end);
    ans.push_back(tmp);
    return ans;
  }
};