if-else 中超过时间限制

Question

Following is a function which change the value of array according to conditions:

1. If the difference between the array element and the next multiple of 5 is less than 3, round up to the next multiple of 5.
2. If the value of array element is less than 38, no rounding occurs.

Code:

vector<int> gradingStudents(vector<int> grades) {
    int n = grades.size();
    int i = 0;
    
    while(i<n)
    {
        if(grades.at(i)<38) 
        {
            i++;
        }
        else if( (grades.at(i)+1) % 5==0 ) 
        {
           grades.at(i) += 1;
           i++;
        }
        else if( (grades.at(i)+2) % 5 == 0 && grades.at(i)>=38) 
        {
            grades.at(i) += 2 ;
            i++;
        }
    }
    
    return grades;

}

Hackerrank site saying " Time Limit Exceeded". I don't understand how can time limit exceeded when there is no loop (except the essential while loop). If I remove i++ from each if statement and put it outside all if statements then its working fine, but the no. of statements remain same. please help me out

Answer 1

Your code only increments i when one of the conditions is met. It is not clear why you are doing this, because no matter what case the number i is, in the next iteration you want to check the next number not i again. Your conditions are not complete, in the sense that for example i=42 does not match any condition. Hence, your loop will get stuck once it encounters the first such number.

Your current issue of an potentially infinte loop can be solved via

while(i<n)
{
    if(grades.at(i)<38) 
    {
    }
    else if( (grades.at(i)+1) % 5==0 ) 
    {
       grades.at(i) += 1;
    }
    else if( (grades.at(i)+2) % 5 == 0) 
    {
        grades.at(i) += 2 ;
    }
    i++;
}

Or the equivalent for loop.

Answer 2

Time Limit Exceeded happens most often when the input size if very large, and/ or your logic takes a long time to process. One place I see in your code is, if there was a place where no if statements were satisfied, it will never be because i wont be incremented. Basically your loop will run forever.

You can either increment i in the end of the while loop (because anyway you will increment it),

while(i<n)
{
    if(grades.at(i)<38) 
    {
    }
    else if( (grades.at(i)+1) % 5==0 ) 
       grades.at(i) += 1;
    else if( (grades.at(i)+2) % 5 == 0) 
        grades.at(i) += 2;

    i++;
}

Or use a for loop,

for(int i = 0; i < n; i++)
{
    if(grades.at(i)<38) 
    {
    }
    else if( (grades.at(i)+1) % 5==0 ) 
       grades.at(i) += 1;
    else if( (grades.at(i)+2) % 5 == 0) 
        grades.at(i) += 2;
}