返回 str 与 str.substr(0,str.size()) 在 leetcode 中给了我不同的输出

Question

I just solved https://leetcode.com/problems/push-dominoes/ .

My code is

class Solution {
public:

    string pushDominoes(string dominoes) {
        string res(dominoes.size(), ' ');
        dominoes = 'L' + dominoes + 'R';

        int l = 0;
        for(int r = 1; r < dominoes.size(); r++)
        {
            if(dominoes[r] != '.')
            {
                //check which case we have [L...R], [R....L] [L.....L]  [R...R]
                if(l != 0) res[l-1] = dominoes[l]; 
                if(r != dominoes.size() - 1) res[r-1] = dominoes[r];

                if(dominoes[l] == dominoes[r])
                {
                    for(int i = l; i <= r; i++) 
                        if(i > 0) res[i-1] = dominoes[r];
                }
                else if(dominoes[l] == 'L' && dominoes[r] == 'R')
                    for(int i = l+1; i < r; i++) res[i-1] = '.';
                else if(dominoes[l] == 'R' && dominoes[r] == 'L')
                {
                    if((l+r)%2 == 0) 
                    {
                        auto mid = (l+r)/2;
                        res[mid-1] = '.';
                        for(int i = l+1; i < mid; i++) res[i-1] = 'R';
                        for(int i = mid+1; i < r; i++) res[i-1] = 'L';
                    }
                    else
                    {
                        auto mid = (l+r)/2;
                        for(int i = l+1; i <= mid; i++) res[i-1] = 'R';
                        for(int i = mid+1; i < r; i++) res[i-1] = 'L';
                    }
                }

                l = r;
            }
        }

        return res;
        // return res.substr(0, res.size());
    }
};

int main( ) 
{ 
   Solution soln;

   cout << soln.pushDominoes(".L.R.") << endl;
}

For one of the test cases, where the input is ".LR" leetcode states that my output is "LL.RRRLLRRLL.." when I use return res . The answer is supposed to be "LL.RR". But I printed out res and it's indeed "LL.RR" and size 5.

If I change my code to return return res.substr(0, res.size()) instead of return res , I get the correct solution. I'm perplexed as to why this is occuring when these 2 return statements should be identical?

I also ported the code over and compiled it on my computer, and I get the correct solution. This makes me wonder if there's some differences in how my code is handled between different C++ compilers, or if Leetcode might have some issues for this particular problem. Any advice?

Answer 1

string res(dominoes.size(), ' ');

Sets the size of res to that of dominoes and this size is never changed. res can never be larger than dominoes . With input of ".LR", dominoes 's length is 5 . res is length 5.

dominoes = 'L' + dominoes + 'R';

Changes the size of dominoes to 7.

for (int r = 1; r < dominoes.size(); r++)

iterates r from 1 to 6.

Aside: When ever I see a <= in a for loop I stop for a longer look. It's wrong far more often than it is right.

In this case it seems to be only sort of wrong, allowing

for (int i = l; i <= r; i++)
    if (i > 0) res[i - 1] = dominoes[r];

to iterate i from l to r . Since r can be 6, i can be 6. That means

    if (6 > 0) res[6 - 1] = dominoes[6];

is possible. This resolves to

    res[5] = dominoes[6];

and res[5] is not valid. Writing there invokes undefined behaviour, and in this case it appears to overwrite the null terminator of the c-style string backing the string .

Another aside: Avoid using l as a variable name. It looks too much like 1 and leads to bugs or misreading of the code.

To fix, you could expand the logic of if (i > 0) into if (i > 0 && i-1 < res.size()) to filter out this problem, but you're better off reworking or replacing the algorithm so that you never find yourself in this situation.