Question

1759. Count Number of Homogenous Substrings

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Description

Given a string s, return _the number of homogenous substrings of _s_._ Since the answer may be too large, return it modulo 109 + 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 <= 105
• s consists of lowercase letters.

Solutions

Solution 1

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

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 {
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;
  }
}

int countHomogenous(char* s) {
  int MOD = 1e9 + 7;
  int ans = 0;
  for (int i = 0, j = 0; s[j]; j++) {
    if (s[i] != s[j]) {
      i = j;
    }
    ans = (ans + j - i + 1) % MOD;
  }
  return ans;
}

Solution 2

class Solution:
  def countHomogenous(self, s: str) -> int:
    mod = 10**9 + 7
    ans = cnt = 1
    for a, b in pairwise(s):
      cnt = cnt + 1 if a == b else 1
      ans = (ans + cnt) % mod
    return ans

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

  public int countHomogenous(String s) {
    int n = s.length();
    int ans = 1, cnt = 1;
    for (int i = 1; i < n; ++i) {
      cnt = s.charAt(i) == s.charAt(i - 1) ? cnt + 1 : 1;
      ans = (ans + cnt) % MOD;
    }
    return ans;
  }
}

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

  int countHomogenous(string s) {
    int n = s.size();
    int ans = 1, cnt = 1;
    for (int i = 1; i < n; ++i) {
      cnt = s[i] == s[i - 1] ? cnt + 1 : 1;
      ans = (ans + cnt) % mod;
    }
    return ans;
  }
};

func countHomogenous(s string) int {
  n := len(s)
  const mod int = 1e9 + 7
  ans, cnt := 1, 1
  for i := 1; i < n; i++ {
    if s[i] == s[i-1] {
      cnt++
    } else {
      cnt = 1
    }
    ans = (ans + cnt) % mod
  }
  return ans
}