Smallest index with digit sum equal to index

Array Math

80.1% acceptance

Feb 25, 2026

You are given an integer array nums. Return the smallest index i such that the sum of the digits of nums[i] is equal to i. If no such index exists, return -1.

Solution

Rust

Time O(2^n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn smallest_index(nums: Vec<i32>) -> i32 {
    fn digit_sum(mut x: i32) -> i32 {
      let mut s = 0;
      while x > 0 { s += x % 10; x /= 10; }
      s
    }
    for (i, &v) in nums.iter().enumerate() {
      if digit_sum(v) == i as i32 { return i as i32; }
    }
    -1
  }
}