Sum of increasing product blocks

Math Simulation

69.0% acceptance

Mar 31, 2026

You are given an integer n. A sequence is formed as follows: The 1st block contains 1. The 2nd block contains 2 * 3. The ith block is the product of the next i consecutive integers. Let F(n) be the sum of the first n blocks. Return an integer denoting F(n) modulo 109 + 7.

Solution

Rust

Time O(n²)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn sum_of_blocks(n: i32) -> i32 {
    const MOD: i64 = 1_000_000_007;
    let mut sum = 0i64;
    let mut cur = 1i64;
    for i in 1..=n as i64 {
      let mut prod = 1i64;
      for _ in 0..i {
        prod = prod * cur % MOD;
        cur += 1;
      }
      sum = (sum + prod) % MOD;
    }
    sum as i32
  }
}