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Formatted question description: https://leetcode.ca/all/1759.html

1759. Count Number of Homogenous Substrings

Level

Medium

Description

Given a string s, return the number of homogenous substrings of s. Since the answer may be too large, return it modulo 10^9 + 7.

A string is homogenous if all the characters of the string are the same.

A substring is a contiguous sequence of characters within a string.

Example 1:

Input: s = “abbcccaa”

Output: 13

Explanation: The homogenous substrings are listed as below:

“a” appears 3 times.

“aa” appears 1 time.

“b” appears 2 times.

“bb” appears 1 time.

“c” appears 3 times.

“cc” appears 2 times.

“ccc” appears 1 time.

3 + 1 + 2 + 1 + 3 + 2 + 1 = 13.

Example 2:

Input: s = “xy”

Output: 2

Explanation: The homogenous substrings are “x” and “y”.

Example 3:

Input: s = “zzzzz”

Output: 15

Constraints:

  • 1 <= s.length <= 10^5
  • s consists of lowercase letters.

Solution

First, find all longest substrings such that all characters in the substring are the same. Next, calculate the count of each longest substring. If a longest substring have length n, then the number of homogeneous substrings in such a substring is n * (n + 1) / 2. Finally, return the total count.

  • class Solution {
    public int countHomogenous(String s) {
        final int MODULO = 1000000007;
        long totalCount = 0;
        int length = s.length();
        char prev = '0';
        int consecutive = 0;
        for (int i = 0; i < length; i++) {
            char c = s.charAt(i);
            if (c == prev)
                consecutive++;
            else {
                long curCount = (long) consecutive * (consecutive + 1) / 2 % MODULO;
                totalCount = (totalCount + curCount) % MODULO;
                consecutive = 1;
                prev = c;
            }
        }
        long curCount = (long) consecutive * (consecutive + 1) / 2 % MODULO;
        totalCount = (totalCount + curCount) % MODULO;
        return (int) totalCount;
    }
}

############

class Solution {
    private static final int MOD = (int) 1e9 + 7;

    public int countHomogenous(String s) {
        int n = s.length();
        long ans = 0;
        for (int i = 0, j = 0; i < n; i = j) {
            j = i;
            while (j < n && s.charAt(j) == s.charAt(i)) {
                ++j;
            }
            int cnt = j - i;
            ans += (long) (1 + cnt) * cnt / 2;
            ans %= MOD;
        }
        return (int) ans;
    }
}

  • class Solution:
    def countHomogenous(self, s: str) -> int:
        mod = 10**9 + 7
        i, n = 0, len(s)
        ans = 0
        while i < n:
            j = i
            while j < n and s[j] == s[i]:
                j += 1
            cnt = j - i
            ans += (1 + cnt) * cnt // 2
            ans %= mod
            i = j
        return ans

############

# 1759. Count Number of Homogenous Substrings
# https://leetcode.com/problems/count-number-of-homogenous-substrings

class Solution:
    def countHomogenous(self, s: str) -> int:
        n = len(s)
        M = 10 ** 9 + 7
        res = 0
        
        mp = Counter(s)
        
        i = 0
        while i < n:
            j = i + 1
            while j < n and s[i] == s[j]:
                res += j - i
                j += 1
            
            i = j
        
        return (res + sum(mp.values())) % M


  • class Solution {
public:
    const int mod = 1e9 + 7;

    int countHomogenous(string s) {
        int n = s.size();
        long ans = 0;
        for (int i = 0, j = 0; i < n; i = j) {
            j = i;
            while (j < n && s[j] == s[i]) ++j;
            int cnt = j - i;
            ans += 1ll * (1 + cnt) * cnt / 2;
            ans %= mod;
        }
        return ans;
    }
};

  • func countHomogenous(s string) (ans int) {
	n := len(s)
	const mod int = 1e9 + 7
	for i, j := 0, 0; i < n; i = j {
		j = i
		for j < n && s[j] == s[i] {
			j++
		}
		cnt := j - i
		ans += (1 + cnt) * cnt / 2
		ans %= mod
	}
	return
}

  • function countHomogenous(s: string): number {
    const mod = 1e9 + 7;
    const n = s.length;
    let ans = 0;
    for (let i = 0, j = 0; j < n; j++) {
        if (s[i] !== s[j]) {
            i = j;
        }
        ans = (ans + j - i + 1) % mod;
    }
    return ans;
}


  • impl Solution {
    pub fn count_homogenous(s: String) -> i32 {
        const MOD: usize = 1e9 as usize + 7;
        let s = s.as_bytes();
        let n = s.len();
        let mut ans = 0;
        let mut i = 0;
        for j in 0..n {
            if s[i] != s[j] {
                i = j;
            }
            ans = (ans + j - i + 1) % MOD;
        }
        ans as i32
    }
}


  • public class Solution {
    public int CountHomogenous(string s) {
        long MOD = 1000000007;
        long ans = 0;
        for (int i = 0, j = 0; i < s.Length; i = j) {
            j = i;
            while (j < s.Length && s[j] == s[i]) {
                ++j;
            }
            int cnt = j - i;
            ans += (long) (1 + cnt) * cnt / 2;
            ans %= MOD;
        }
        return (int) ans;
    }
}

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