请问这段代码的时间复杂度为什么是O(1)?

Question

bool Increment(char* number)
{
    bool isOverflow = false;
    int nTakeOver=0;
    int nLength = strlen(number);
    for(int i = nLength - 1;i >= 0; i--)
    {
        int nSum = number[i] - '0' + nTakeOver;
        if(i == nLength - 1)
            nSum ++;
        if(nSum >= 10)
        {
            if(i == 0)
            {
                isOverflow = true;
            else
            {
                nSum -=10;
                nTakeOver = 1;
                number[i] = '0' + nSum;
            }
        }
        else{
            number[i] = '0' + nSum;
            break;
        }
    }
    return isOverflow;
}