First Index Equal to Its Value's Digit Sum

# First Index Equal to Its Value's Digit Sum You are given an integer array `nums`, indexed from zero. Return the smallest index `i` for which the sum of the decimal digits of `nums[i]` equals `i`. If no index satisfies this condition, return `-1`. ### C++ Interface ```cpp class Solution { public: int smallestIndex(vector<int>& nums); }; ``` ### Examples **Example 1** ```text nums = [0,100,200] Output: 0 ``` The digit sums are `[0,1,2]`, so all three indices satisfy the condition. Index zero is the smallest. **Example 2** ```text nums = [8,12,90,4] Output: -1 ``` The digit sums are `[8,3,9,4]`. None equals its index in `[0,1,2,3]`. ### Constraints - `1 <= nums.length <= 100`. - `0 <= nums[i] <= 1000`. ### Clarifying Questions - What is the digit sum of zero? It is zero. - If several indices match, which one is returned? The smallest original index. - Does the function return the value at the matching index? No. It returns the index itself. - Are indices one-based? No; the first array position has index zero. ### What a Strong Answer Covers - Exact decimal digit sums, including values with zeros and the value zero itself. - The smallest matching original index when several positions qualify. - The required `-1` result when no position qualifies. - Time and space bounds for at most 100 values, each no larger than 1000. ### Follow-up Questions - If the array contains a single element, which allowed value makes the answer zero? - Can two different values at different positions both satisfy the condition? Give an example. ```hint Check indices in order Compute each value's decimal digit sum without changing the input array. Check positions from zero upward so the first match is already the smallest. ```