Find triangular sum of an array

Array Math Simulation Combinatorics Number Theory

82.0% acceptance

Feb 25, 2026

1540

You are given a 0-indexed integer array nums, where nums[i] is a digit between 0 and 9 (inclusive). The triangular sum of nums is the value of the only element present in nums after the following process terminates: Let nums comprise of n elements. If n == 1, end the process. Otherwise, create a new 0-indexed integer array newNums of length n - 1. For each index i, where 0 <= i < n - 1, assign the value of newNums[i] as (nums[i] + nums[i+1]) % 10, where % denotes modulo operator. Replace the array nums with newNums. Repeat the entire process starting from step 1. Return the triangular sum of nums.

Solution

Rust

Time O(n²)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn triangular_sum(mut nums: Vec<i32>) -> i32 {
    while nums.len() > 1 {
      let n = nums.len();
      for i in 0..n - 1 {
        nums[i] = (nums[i] + nums[i + 1]) % 10;
      }
      nums.truncate(n - 1);
    }
    nums[0]
  }
}