Number of substrings with only 1s

Math String

57.4% acceptance

Feb 25, 2026

1231

Given a binary string s, return the number of substrings with all characters 1's. Since the answer may be too large, return it modulo 109 + 7.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn num_sub(s: String) -> i32 {
    const MOD: i64 = 1_000_000_007;
    let mut ans = 0i64;
    let mut cnt = 0i64;
    for c in s.chars() {
      if c == '1' {
        cnt += 1;
        ans = (ans + cnt) % MOD;
      } else {
        cnt = 0;
      }
    }
    ans as i32
  }
}