反转字符串。 不知道为什么这个逻辑是错误的。 C ++

Question

I am a beginner in solving algorithmic questions. Until now, I have only self-taught coding. So, I am not sure about the proper conventions.

I was trying to solve a question to reverse a string.There is some problem with the code but I am not sure what it is after debugging step-by-step.

class Solution {
public:
    string reverseString(string s) {
        int n = s.length();
        string reverse;
        for (int i=0;i<s.length();i++)
        {
            reverse[i] = s[n-1];
            n=n-1;
        }
        return reverse;
    }
};

Input: "Hello" Output needed: "olleh" My output: "olleh " (extra space)

Input: A man, a plan, a canal: Panama Output: No output

I searched online for solutions. There were related to pointers. It would be great if someone helped me understand why this logic doesn't work and why using pointers is a better idea.

ALREADY GIVEN. CANNOT CHANGE:

string stringToString(string input) {
    assert(input.length() >= 2);
    string result;
    for (int i = 1; i < input.length() -1; i++) {
        char currentChar = input[i];
        if (input[i] == '\\') {
            char nextChar = input[i+1];
            switch (nextChar) {
                case '\"': result.push_back('\"'); break;
                case '/' : result.push_back('/'); break;
                case '\\': result.push_back('\\'); break;
                case 'b' : result.push_back('\b'); break;
                case 'f' : result.push_back('\f'); break;
                case 'r' : result.push_back('\r'); break;
                case 'n' : result.push_back('\n'); break;
                case 't' : result.push_back('\t'); break;
                default: break;
            }
            i++;
        } else {
          result.push_back(currentChar);
        }
    }
    return result;
}

int main() {
    string line;
    while (getline(cin, line)) {
        string s = stringToString(line);

        string ret = Solution().reverseString(s);

        string out = (ret);
        cout << out << endl;
    }
    return 0;
}

Answer 1

Reversing a string is trivial. Just construct a new one from the reverse iterators:

std::string reverse_str(s.rbegin(), s.rend());

or

std::string reverse_str(s.crbegin(), s.crend());

Here's how I would write your function:

string reverseString(const string& s) {
    return {s.crbegin(), s.crend()};
}

Answer 2

As you create reverse , you have to pass the length of the string as an argument, else the created string will be of size 0. This could look like this:

string reverseString(string s) {
    int n = s.length();
    string reverse(n,'0');
    for (int i=0;i<s.length();i++)
    {
        reverse[i] = s[n-1];
        n=n-1;
    }
    return reverse;
}

Answer 3

Try this out

class Solution {
public:
string reverseString(string s) {
    //cout<<"inside func";
    int n = s.length();
    cout<<n<<endl;
    char reverse[sizeof(char)*n];// reverse stores the reverse of original string  s
    int i= 0;
    for ( i=0;i<s.length();i++)
    {

        reverse[i] = s[n-i-1];

    }

    return reverse;
  }
}

int main()
{
  string s,r;
  Solution sol;
  s= "hello";
  r= sol.reverseString(s);
  cout<<r<<endl;
  cout<<r.length()<<endl;
  return 0;
}

when i= 0, ni-1= n-1 which is the last element of the original string s. So the first element of the reverse string is the last element of s. Next i becomes i+1 ie 1. This time second element of the reverse string is the last but one element in string s. This procedure is repeated till i < s.length(). The element to get copied is for index i= n-1 and n becomes n-(n-1)-1= 0, so the last element of reverse string is the first element of s. After this the loop exists. No additional extra characters are added.