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3550. Smallest Index With Digit Sum Equal to Index 👍

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  int smallestIndex(vector<int>& nums) {
    for (int i = 0; i < nums.size(); ++i)
      if (getDigitSum(nums[i]) == i)
        return i;
    return -1;
  }

 private:
  int getDigitSum(int num) {
    int sum = 0;
    while (num > 0) {
      sum += num % 10;
      num /= 10;
    }
    return sum;
  }
};

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class Solution {
  public int smallestIndex(int[] nums) {
    for (int i = 0; i < nums.length; ++i)
      if (getDigitSum(nums[i]) == i)
        return i;
    return -1;
  }

  private int getDigitSum(int num) {
    int sum = 0;
    while (num > 0) {
      sum += num % 10;
      num /= 10;
    }
    return sum;
  }
}

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class Solution:
  def smallestIndex(self, nums: List[int]) -> int:
    return next((i
                 for i, num in enumerate(nums)
                 if self._getDigitSum(num) == i), -1)

  def _getDigitSum(self, num: int) -> int:
    return sum(int(digit) for digit in str(num))